You can use mobile bridge-mode.
It can be set in the Network → Interfaces page by clicking 'Edit' next to the mobile interface. Then under 'General Setup' set Mode to 'Bridge', enter the router's MAC address, save the changes and connect the two devices.
Or you can use Passthrough mode to achieve the same while maintaining more of the gateway's capabilities (which may prove useful if you want to change the TRB's configuration in the future, as setting it to Bridge mode will disable the DHCP server on LAN).
Some additional info on this can be seen here.