Hello,
Are you trying to use the MTR as a Serial Modem or what is your purpose for sending AT commands to the MTR? If not as a Serial Modem, please provide more details.
If trying to use the MTR as a Serial Modem, I would set to Serial Modem Mode. After save and restart, I would open your terminal program directly to the COM port provided by the USB adapter. For example, TeraTerm to COM1. Default settings are 115200 8N1, flow control shouldn’t matter for this initial testing. I would try to interactively communicate with the MTR. For example, issue
ati3 <CR> and receive
Telit
OK
ati4 <CR> and receive
LE910-xxx
OK
Once you know that’s working then that’s when you can work on your ‘echo’ing and ‘cat’ting. We need to first verify your serial setup is working.
Regards.