I understand driving to a different area will change the frequencies, but when I do the test from my desk I am not connected to a satellite, therefore It is not searching for FM frequencies. By hitting reset it should clear the FM frequencies, then do a new search when I am connected to satellites in the car.
Your 1390 differs from the 765T, you only have one power connector, that is the USB connector, THIS WILL NOT POWER YOUR GTM25.
The 765T has a power connector on it’s mount, this is where the GTM20 commects. There is an additional USB port, the USB port is where the home charger plugs in this is senfs power to the GTM20. This is how I’m able to work from my desk. It performs all functions as if I’m connected to the cars power supply, only difference is no satellite signals.