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We have some customers who were able to configure their ASUS DSLAC68U with TPG NBN.
You need to make sure that the modem is configured with VLAN ID '2' and PPPoE (With TPG username and password).
If it's still not working, you may need to contact ASUS support for further checking.
If you're still trying to have this resolved check out the solution posted by @bigRED in the following thread.
I followed the method over the weekend and finally have my DSL-AC68U connected to TPG FTTC. You'll end up with 3 devices connected (NBN modem, TPG router acting as a bridge and then ASUS) but will at least have a decent router connected.
This worked with HFC using VLAN ID 2. In theory, it should work with FTTC if all that is needed is to set the router to VLAN ID 2.