MKR GSM 1400 Works only when connected to computer My MKR GSM 1400 works fine when connected to a computer (powered USB) and after opening a serial connection to it.  It does not when I plug it into a power supply. My power supply is a 5V 3A with a USB adapter.
If your sketch on an Arduino with native USB doesn't run if not connected to USB, always first check if you didn't forget the while (!Serial) {} in setup().