You will find this extremely difficult to accomplish.
I am guessing your NBN is FTTN but please correct me if I am wrong.
And your gateway max is TG799AC .. (Telstra name lots of different routers gateway max to make life easier for support!!)
Are you getting your phone connection via VOIP through the gateway. You cannot bridge it in that case.
What that all adds up to is a big NO for bridging the gateway. If you actually do not need the voip then it might be possible to bridge but this is not easy. Also the fact is the TC does not tend to work well in this setup.
There are a couple of methods to use the TC in conjunction with the gateway.. but using it as a router is not great.
I wish to use the TC as my router and for backups
Backups are no problem. Any setup tends to work ok.
Using the TC as router might simply not work or work badly.
Tell me why you want to use the TC as your router.. ??
I use a TC in static IP mode with DHCP working.. that means all the clients of the TC get their connection via the TC and it is a partial router, but no NAT.. this works extremely well and I would commend it as way around the issue.
See https://discussions.apple.com/thread/5981989?answerId=25135547022#25135547022&ac_cid=tw123456#
Some people do keep using the TG799 as main router and also use the TC as router in the DMZ. That also can work ok.. although IMHO using static IP simply works better.