ok, which GPIOs are you using for TX and RX? M5Stack Fire uses GPIO 16 and 17 internally, so you'll need to switch to a different pair of GPIOs.
From the view of the COM.GSM module TX can be 16, 5 or 15 and RX can be 17, 0 or 13.
Whatever pair you choose you'll also need to adapt those two lines in the sketch:
#define TX_PIN 16 #define RX_PIN 17