Configuring the WAN port as additional LAN port gives the same result. Device connected there ahs no connectivity, not getting a DHCP lease etc.
Both tcpdumps show nothing (not a single packet) after running for 10 minutes each.
The log from the RUT 240 still is showing those packets with its' own address as source (00:1e:42:42:9e:74 is the MAC of eth0, which is seen on eth1 - this is with WAN configures as LAN, i.e. eth0, eth1 and wlan0 in br-lan).
Wed Oct 5 16:59:52 2022 daemon.notice netifd: mob1s1a1_4 (25952): udhcpc: started, v1.34.1
Wed Oct 5 16:59:52 2022 daemon.notice netifd: mob1s1a1_4 (25952): udhcpc: broadcasting discover
Wed Oct 5 16:59:52 2022 daemon.notice netifd: mob1s1a1_4 (25952): udhcpc: broadcasting select for 100.124.242.73, server 100.124.242.74
Wed Oct 5 16:59:52 2022 daemon.notice netifd: mob1s1a1_4 (25952): udhcpc: lease of 100.124.242.73 obtained from 100.124.242.74, lease time 7200
Wed Oct 5 16:59:53 2022 daemon.notice netifd: Interface 'mob1s1a1_4' is now up
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: reading /tmp/resolv.conf.d/resolv.conf.auto
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using only locally-known addresses for domain test
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using only locally-known addresses for domain onion
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using only locally-known addresses for domain localhost
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using only locally-known addresses for domain local
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using only locally-known addresses for domain invalid
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using only locally-known addresses for domain bind
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using only locally-known addresses for domain lan
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using nameserver 139.7.30.125#53
Wed Oct 5 16:59:53 2022 daemon.info dnsmasq[3883]: using nameserver 139.7.30.126#53
Wed Oct 5 16:59:54 2022 user.warn mwan3-hotplug[25913]: hotplug called on mob1s1a1 before mwan3 has been set up
Wed Oct 5 16:59:55 2022 daemon.info ledman[4143]: Failed to lookup for network.interface.wan object
Wed Oct 5 16:59:55 2022 daemon.info ledman[4143]: Failed to lookup for network.interface.wan6 object
Wed Oct 5 16:59:57 2022 user.notice firewall: Reloading firewall due to ifup of mob1s1a1 (wwan0)
Wed Oct 5 17:00:01 2022 user.notice dnsmasq: dhcp_check(): using cached value: 0
Wed Oct 5 17:00:02 2022 daemon.info dnsmasq[3883]: read /etc/hosts - 4 addresses
Wed Oct 5 17:00:02 2022 daemon.info dnsmasq[3883]: read /tmp/hosts/dhcp.cfg01411c - 2 addresses
Wed Oct 5 17:00:02 2022 daemon.info dnsmasq-dhcp[3883]: read /etc/ethers - 0 addresses
Wed Oct 5 17:00:03 2022 daemon.info unhandler[1927]: *Unsolicited id: 1-1 | msg: '+CREG: 1,"BBB2","14ABF08",7'
Wed Oct 5 17:00:03 2022 daemon.info unhandler[1927]: *Unsolicited id: 1-1 | msg: '+CGREG: 1,"BBB2","14ABF08",7'
Wed Oct 5 17:00:03 2022 user.warn mwan3-hotplug[26475]: hotplug called on mob1s1a1_4 before mwan3 has been set up
Wed Oct 5 17:00:04 2022 daemon.info unhandler[1927]: *Unsolicited id: 1-1 | msg: '+CREG: 1,"BBB2","14ABF15",7'
Wed Oct 5 17:00:04 2022 daemon.info unhandler[1927]: *Unsolicited id: 1-1 | msg: '+CGREG: 1,"BBB2","14ABF15",7'
Wed Oct 5 17:00:04 2022 user.notice firewall: Reloading firewall due to ifup of mob1s1a1_4 (wwan0)
Wed Oct 5 17:00:07 2022 user.notice dnsmasq: dhcp_check(): using cached value: 0
Wed Oct 5 17:00:08 2022 daemon.info dnsmasq[3883]: read /etc/hosts - 4 addresses
Wed Oct 5 17:00:08 2022 daemon.info dnsmasq[3883]: read /tmp/hosts/dhcp.cfg01411c - 2 addresses
Wed Oct 5 17:00:08 2022 daemon.info dnsmasq-dhcp[3883]: read /etc/ethers - 0 addresses
Wed Oct 5 17:00:09 2022 kern.info Mobile data connected (internal modem)
Wed Oct 5 17:00:09 2022 kern.info Joined FDD LTE network (internal modem)
Wed Oct 5 17:00:09 2022 kern.info Connected to vodafone.de 1&1 operator (internal modem)
Wed Oct 5 17:00:13 2022 daemon.info hostapd: wlan0: STA 5c:80:b6:70:cf:d3 IEEE 802.11: authenticated
Wed Oct 5 17:00:13 2022 daemon.info hostapd: wlan0: STA 5c:80:b6:70:cf:d3 IEEE 802.11: associated (aid 1)
Wed Oct 5 17:00:13 2022 daemon.notice hostapd: wlan0: AP-STA-CONNECTED 5c:80:b6:70:cf:d3
Wed Oct 5 17:00:13 2022 kern.notice WiFi client connected: 5C:80:B6:70:CF:D3
Wed Oct 5 17:00:13 2022 daemon.info hostapd: wlan0: STA 5c:80:b6:70:cf:d3 RADIUS: starting accounting session C78C33E31D2EDA68
Wed Oct 5 17:00:13 2022 daemon.info hostapd: wlan0: STA 5c:80:b6:70:cf:d3 WPA: pairwise key handshake completed (RSN)
Wed Oct 5 17:00:14 2022 kern.warn kernel: [ 7683.074583] net_ratelimit: 27 callbacks suppressed
Wed Oct 5 17:00:14 2022 kern.warn kernel: [ 7683.074606] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.notice Authentication was successful from HTTP 192.168.1.17
Wed Oct 5 17:00:22 2022 daemon.err uhttpd[2331]: vuci: accepted login for admin from 192.168.1.17
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7690.963466] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.062411] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.071279] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.081195] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.091227] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.104982] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.113728] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.130732] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.139494] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:22 2022 kern.warn kernel: [ 7691.149503] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:28 2022 kern.warn kernel: [ 7697.666372] net_ratelimit: 5 callbacks suppressed
Wed Oct 5 17:00:28 2022 kern.warn kernel: [ 7697.666394] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.notice Authentication was successful from HTTP 192.168.1.17
Wed Oct 5 17:00:31 2022 daemon.err uhttpd[2331]: vuci: accepted login for admin from 192.168.1.17
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7699.852206] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7699.955366] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7699.964279] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7699.974157] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7699.984221] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7699.994253] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7700.004319] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7700.018134] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:31 2022 kern.warn kernel: [ 7700.026847] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:34 2022 kern.warn kernel: [ 7703.107295] net_ratelimit: 6 callbacks suppressed
Wed Oct 5 17:00:34 2022 kern.warn kernel: [ 7703.107317] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:35 2022 kern.warn kernel: [ 7704.578665] br-lan: received packet on eth1 with own address as source address (addr:00:1e:42:42:9e:74, vlan:0)
Wed Oct 5 17:00:44 2022 daemon.info unhandler[1927]: *Unsolicited id: 1-1 | msg: '+CREG: 2'
Wed Oct 5 17:00:44 2022 daemon.info unhandler[1927]: *Unsolicited id: 1-1 | msg: '+CGREG: 2'
Wed Oct 5 17:00:45 2022 kern.notice Authentication was successful from HTTP 192.168.1.17
Wed Oct 5 17:00:45 2022 daemon.err uhttpd[2331]: vuci: accepted login for admin from 192.168.1.17
Wed Oct 5 17:00:53 2022 user.notice qmux_track: Connection lost.