Little update.

I am able to receive the initialization message from the SIM7600 board, so at least I know that the hardware serial read works :

RDY +CPIN: READY SMS DONE PB DONE

If anyone has this issue, you can just do a read in the main loop....

String response = ""; while (SIMCOM.available()) { char c = SIMCOM.read(); response += c; } Serial.print(response);

and then power cycle the SIM7600 with The T-Display-S3 connected

Still a mystery as to what is going on