How to set the APN for GSM /SMS texting?

Hi

As part of the initialisation routine, tinyGSM does reset the Modem
If you look down the sketch you'll see the line "if (!modem.restart())"

This issues the command AT&FZE0
This is the same as issuing the following three commands

AT&F <== Factory reset
ATZ <== Load User profile (If the profile has never been saved with AT&W, it is the same as usibg AT&F)
ATE0 <== Turn local echo off

If you wish, you could put in the additional lines just before the line "String modemInfo = modem.getModemInfo();"

  DBG("Resetting to factory default");
  modem.sendAT(GF("&F"));  // Factory default
  if (modem.waitResponse() != 1) {
    DBG("Could not set to factory defaults");
  }
  DBG("Set to factory defaults");

Did you receive a call from the device on your mobile at all?
The test seemed to indicate that you did