Hi @rmklallen . The Arris provides the physical and protocol interfaces into the NBN HFC network from ethernet data streams. The wifi router manages ethernet devices and wifi devices and landline telephone into ethernet streams to the Arris. It's a division of labour.
You have the option of changing the TPLink router to a better one (faster wifi, better coverage, more devices). For your HFC connection, the router has to support "VLAN ID=2" setting. The TPG VOIP phone function only works with the TPG supplied router.