I'm using a RUT240 (with firmware RUT2XX_R_00.01.12.1) in bridge mode: connected to the LAN port of the RUT240 is the WAN port of an OpenWrt router. I specified the MAC of OpenWrt's WAN ethernet port under "Bind to MAC". There's no other device connected to the RUT240, WiFi is disabled. Therefore the RUT240 is just operating as a LTE modem. There's no need for any other functionality.
Now I found under Network -> WAN -> Edit Mobile WAN -> Advanced Settings a check box "Disable NAT", which is unchecked.
I thought, when enabling bridge mode NAT is automatically disabled (because it's not necessary). Or do I have to explicitly disable NAT.
Thanks for reading and answering.