Welcome to the Community!
Customers can use their third party modem/router as long as it is not locked with your previous provider.
If you are on a BYO NBN plan, then you need to configure the connection type to PPPoE and use your TPG username and password. (no need to set the VLAN ID to 2).
If you are under the NBN bundled plan, then the VoIP will not work with the third party device and you need to set the VLAN ID to 2 along with the PPPoE.