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Power supply voltage measurement is not connected to processor GPIO pin, instead it uses LTE module ADC input 0. You can read this value with command gmsctl -A AT+QADC=0. This will return +QADC: <status>,<value>. Here <status> indicates if ADC value was read successfully: 0 - Fail, 1 - Success, and <value> is ADC voltage value in mV. However, PSU voltage is not directly measured, instead it goes via voltage divider which ratio is 14.7/164.7. So to get PSU voltage you have to calculate it with formula: PSU_voltage = ADC_voltage * 164.7/14.7.
What is your output of the command below:
gsmctl -m -y