Had a great deal of trouble at one site, trying to replace the dreadful TP-Link Archer 1600 device supplied by TPG. There’s lots of instructions available but these mostly concentrate on VDSL connections. My customer has HFC NBN. Anyway, the answer was relatively simple once I’d found a way to turn on both PPPOe and VLAN on for the WAN connection

Here’s a pic- note VLAN 2 set at the bottom, the whole email address is used for Username, and ‘noPassword’ is the Password.