Wwan 4G/LTE in bridge mode

I bought a TP-Link MR200 with the intention of wiping the firmware and replacing with OpenWRT - it seemed it was the only easily available (locally) and compatible device with 4G/LTE. However, I didn't read the fineprint, and the hardware is a v4 - unsupported!

Is it possible to use the device in bridge mode?

Could I set it to say, let it establish the connection, and then set it as the gateway for the whole LAN ( via my existing RPi3 openWRT 19.07.3 router?
Would the LTE device be connected via its WAN ethernet port, or just via std ports?
Is the an example of the layout/settings that I can look up and "copy"?