Hi
You need to check the ignition source of your vehicle. kindly check this link
FMB920 System settings - Wiki Knowledge Base | Teltonika GPS (teltonika-gps.com)
maybe in your configuration, you choose the POWER VOLTAGE as your ignition source and you didn't set the High voltage and the Low voltage according to the battery of your vehicle.
In this case, you can select the DIN 1 as your ignition source, connect it to the ignition of your vehicle, take note that the ignition wire is different from the power wire, as the ignition wire turns into 0 volts when the ignition is off. In this setting, no need to check the battery voltage of your vehicle as the ignition status depends on the ignition wire.
Regards
Maynard