Thank you, I could use tcpdump on my router following your informations.
If I dump the ethernet interface, I see the broadcast UDP frames sent by device 192.168.17.80, as in the following excerpt:
04:56:54.559079 IP Teltonika.lan.www > 192.168.17.80.37284: Flags [.], ack 75, win 14600, length 0
04:56:54.577480 IP 192.168.17.80.8001 > 192.168.17.255.9002: UDP, length 40
04:56:55.078359 ARP, Request who-has Teltonika.lan tell 192.168.17.80, length 46
04:56:55.078748 ARP, Reply Teltonika.lan is-at 00:1e:42:12:61:cc (oui Unknown), length 28
04:56:55.079129 IP 192.168.17.80.37285 > Teltonika.lan.www: Flags [S], seq 1818744915, win 2048, options [mss 1460], length 0
04:56:55.079912 IP Teltonika.lan.www > 192.168.17.80.37285: Flags [S.], seq 3041044891, ack 1818744916, win 14600, options [mss 1460], length 0
04:56:55.080292 IP 192.168.17.80.37285 > Teltonika.lan.www: Flags [.], ack 1, win 2048, length 0
04:56:55.113096 IP 192.168.17.80.37285 > Teltonika.lan.www: Flags [P.], seq 1:74, ack 1, win 2048, length 73
04:56:55.113683 IP Teltonika.lan.www > 192.168.17.80.37285: Flags [.], ack 74, win 14600, length 0
04:56:55.247369 IP Teltonika.lan.www > 192.168.17.80.37285: Flags [P.], seq 1:37, ack 74, win 14600, length 36
04:56:55.248255 IP Teltonika.lan.www > 192.168.17.80.37285: Flags [FP.], seq 37:123, ack 74, win 14600, length 86
04:56:55.248741 IP 192.168.17.80.37285 > Teltonika.lan.www: Flags [.], ack 124, win 1926, length 0
04:56:55.618607 IP 192.168.17.80.37285 > Teltonika.lan.www: Flags [F.], seq 74, ack 124, win 2048, length 0
04:56:55.619175 IP Teltonika.lan.www > 192.168.17.80.37285: Flags [.], ack 75, win 14600, length 0
04:56:55.637545 IP 192.168.17.80.8001 > 192.168.17.255.9002: UDP, length 40
04:56:56.138416 ARP, Request who-has Teltonika.lan tell 192.168.17.80, length 46
(see second line, and last-but-one line). The frames are sent about once a second.
If I capture on the wlan0 interface, I can see no such frames.
This confirms what I had already found using Wireshark connected on the ethernet network, then on the wifi network of the RUT500.
So, why don't the UDP broadcast frames be forwarded on wifi?