Power on the UniFi Secuirty Gateway by plugging it to the power adapter.
Using a pin or paperclip, press and hold the RESET button for 10 seconds. The VOIP LED will begin to flash. Once it becomes solid, you can release the button.
Wait a few minutes while the UniFi Secuirty Gateway restores to the factory default settings.
The default username is ubnt and the default password is ubnt.