Welcome to TPG community!
I can see that the DSL-AC68U is a Dual-band 802.11ac Wi-Fi ADSL/VDSL Modem Router.
It may work given that you configured it with the correct settings such as entering your correct TPG username and Password. However, the homephone feature will not work since the configuration of the VoIP phone is embedded in the modem's firmware.
Understand if you get the ASUS to connect VOIP homephone won't work. If you don't use homephone VOIP but say moblie phone for all your calls not using the router/modem for calls then the ASUS if you get it to work won't be able to make phone calls using voip 1 and 2 ports on back.
I have found the HG659 works really well. It has dual band 2.4 and 5Ghz as well as good range it services our two story house with timber walls really well.
I used to use Billion routers for ADSL but with the NBN the supplied router works great. The only reason you would want to use a custom router would be for wireless range but with the new wireless technology MESH wireless extenders they are no longer needed.
Thanks for sending the screenshot.
Let's try this out.
In WAN - Dual WAN page, please set Primary WAN to Ethernet WAN and choose a proper LAN port which is not used currently. For example, I am using LAN port 3 so I choose LAN port 4. And please press Apply button, wait for the process done.
Then go to WAN - Internet Connection page. Please set WAN Connection Type to PPPoE sometimes, you'll have PPPoE LLC. Please set 802.1Q. Enable field to Yes, and fill the VLAN ID to 2 and 802.1P to 0. Then please press the apply button. Then please retest your connection.
I've also seen in other forums that ASUS currently has an issue in that when you use a LAN port for your WAN port it does not allow you to specify VLAN 2 as it seems to be reserved for the external facing ports on the device. It can probably be fixed with firmware.
Should the issue persists, I would recommend to contact the manufacturer for further assistance.