If you have HFC, you must have a black Arris NBN box which connects to the wall socket with coax cable.
The TPG wifi router (TP-Link Archer?) connects to the Arris.
Do you plan to use the VOIP phone service with NBN? If you do, you have to have the TPG router as the router connected to Arris. If not, you might be able to just use the Covr point A device connected to Arris.
Covr configuration is different in each case.