if you are using Asus DSL-AC68U and like to setup modem with Vlan ID 2 then this modem never goign to work. vLan ID 2 hard coded to use on modem. for this modem you only only use vLan ID 3 and more.
if you call Asus support they will lie and they will told this is TPG issue but this is not.
this is Asus issue and they asking to buy another modem to work with TPG
I have the DSL-AC68R and also had the same issue, but what I did was connect the TPG TP-Link ethernet cable from that router to my Asus router and changed a few bridge settings and it is working correctly now.
However I have the nbn Modem connected to the TP-Link, and then the TP-Link connected to the Asus... it's a bit of an annoying setup but works very well with no speed decreases.