May I ask what is your router module?
is it a VMG3625-T20A device?
And, have you change the admin
password before? If no, please refer to the back label value to test first. If
the back label value does not work or you have forgotten what password value you have
changed. Please try to factory reset of your device. You can long push the reset button in the back of the shell until the power led start blink in RED. The device will recover to factory reset
setting, you can enter GUI and re-configuration your device again.