Hi,
To setup a bridge, simply navigate to Network -> Interfaces -> Edit mobile interface (mob1s1a1) -> select bridge mode (or passthrough, in which case the TRB will still be reachable). A new field will appear where you can enter the MAC address of your device to which you want to pass the IP address of the SIM card.
Your device will need to be configured to obtain IP settings automatically (DHCP).
Also, make sure that the DHCP server is enabled on your TRB LAN interface. You can check this if you navigate to Network -> Interfaces -> edit LAN interface.
After that, simply reboot your TRB. Give it a few minutes to fully boot up and connect to the operator. Then, reboot the device to which you are passing the IP address.
You can find more information here and here. Also, there are plenty of threads about the bridge and pass-through mode on this forum, so you can take a look at them too.
Kind Regards,
Andzej