Question, adding a hardware serial UART interface to Arduino MKR GSM 1400

Official Hardware MKR Family MKR GSM 1400
2

Hi season90,

TX should be on digital pin 0 (PA22), as that's on SERCOM3/PAD[0]. Likewise, RX should be on digital pin 1 (PA23):

Uart Serial_plc(&sercom3, 1, 0, SERCOM_RX_PAD_1, UART_TX_PAD_0);

I'd also suggest calling the pinPeripheral() functions before testing the Serial_plc in the while() loop.