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.