Hello @ElwinHi
Please confirm your house extension (a garage/ barn/shed behind the house etc.) and the main building is under the same electrical meter.
Powerline signal strength loses over distance and most house extension has a certain range from the main building, you could try a different Wall socket for the powerline unit plugged in the garage, or move the main unit to a wall socket which is closer to the garage and see if kit stays paired (House Led ON means unit is paired)
The login password is 'admin' or created firstly login PLC in Tether app or web page, you could factory reset wireless powerline unit if login failed:
RESET: with PLC adapter powered on, use a pin to press and hold this Reset button (pin hole)until all the Leds flash once.