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