I used above WiFi router with Optus cable internet without an issue. All I need to do is to connect WAN port on the ASUS to the Internet port on Optus cable modem. With TPG nbn modem, It does not work at all. I had to SW off the WIFI on the tp link router and change Asus router to AP to get it work.
We dont support non TPG modems. Maybe the manufacturer can offer some guidance.
However, if you try that modem and configure it with your TPG USERNAME AND PASSWORD, and make sure a VLAN ID setting is set to VLAN 2, it may work.
With the TPG modem, maybe you can try a factory reset and start again. The wifi should work as normal if the modem is not faulty.
Please find an attached link for a guide to help you configure your modem.
Let us know how you go.