The wifi router sent by TPG connects to the NBN box by ethernet cable.
One consideration is to use a long ethernet cable from the NBN box in garage to router so that the router is inside the house so you can connect your computer to the router directly instead of being all wifi connections. This is ok if your garage is connected to your house, not so much if it is a detached garage. Also depends on how accessible it is between garage and house.
If the NBN box connects into a telephone-type wall socket, the telephone line could be extended into your house. If your connection is HFC or FTTP, it's an expensive exercise.
By the way, what type of connection do you have?
Do you have more than one telephone outlet in your house?
If your NBN box plugs into telephone wall socket (currently in the garage) and you have a telephone socket in the house, try plugging the NBN box into the socket in the house.