Hi @cwu23458 . Settings are PPPoE, TPG username and password. This should be enough for FTTP.
For FTTC and HFC, you need to set VLAN ID=2. This iiNet article describes setting for Asus RT AC68U which has similar menu structure to the AX82U. (Same for FTTC and HFC.)
If you are on BYO data-only plan (no VOIP service), VLAN ID is not used.
@cwu23458 . Have you tried the desktop computer on all 4 LAN ports?
Do you have another ethernet cable?
Open a command window and do the command: ipconfig /all
Copy and paste the ethernet details back here.