Make a forward from one UART to another UART
I am not sure fully about how to implement this "forwarding" (copy like)


An (often quicker) alternative to this type of software "forwarding" approach, is to monitor the two signals you mention, directly via hardware.

I'm short of time, so excuse the lack of a diagram at the moment. In brief:

MCU Tx -&gt; GSM module Rx -> Here connect Rx of UART-to-USB adapter 1
MCU Rx &lt;- GSM module Tx -> Here connect Rx of UART-to-USB adapter 2

Note: You only connect the Rx inputs of those two UART-to-USB adapters. Leave the Tx pins of those adapters unconnected.

Then you can monitor (and, although I've forgotten the name at the moment, I'm sure I've used software to merge or view) these two data streams on a PC. You could even run two simultaneous copies of "Tera Term" as you suggested, one opening the PC COM port for UART-to-USB adapter 1; the other opening the PC COM port for UART-to-USB adapter 2.

This avoids adding complexity at the MCU now. You can add this MCU functionality later, if you need to and when you have learned those programming skills. However during development, using hardware monitoring can be a quicker way to achieve the end result (i.e. monitoring those serial data streams) which you want.