@dima are you having a specific issue?
in the case I refer to here, I am using the LTE Base Shield (https://docs.sixfab.com/docs/raspberry-pi-3g-4g-lte-base-shield-introduction) - on the reverse of the board are two sets of solder pads - one on the UART RX line and the other on the UART TX line; by default (as shipped from the factory) these are open-circuit, thus the Modem sockets UART lines are disconnected from the shields header pins. The implication being that, if you stacked another shield/HAT that required the UART, then that would be fine. In this scenario you must use the USB connection from the Pi to the shield.
If you solder a 0 ohm resistor - or jumper over those pads, then the Pi’s UART RX/TX are now connected to the shields modem socket - you are free to use either UART or the USB. However, the Pi’s UART (pins 14/15) is now taken by the shield, and can’t be used elsewhere (otherwise you will have conflicts)
in my case - i have the LTE Shield and an RS-232 shield - so I have to choose what I want to use. I have specific cases where I want the modem over UART and others when it doesn’t matter, but I need the UART elsewhere, so i would like to put some jumpers on those pads instead of 0 ohm resistors.
but as this thread refers to the LTE Base Shield V2 - it might be worth posting your question to another thread to get the right attention as you refer to a different board