@patrickl That's indeed what I would do: just "interconnect" the UART connected to the Pi with the LTE's UART, sending data receiving from one end to the other and vice versa.
Then the modem will look like it's directly connected to the Pi.
Just don't call LTE and make sure you have the same speed settings everywhere.
An alternative would probably to do it over BLE, but that seems like quite some more work to me.