Seems the firmware for this included AC router is locked.
With my home network setup, I require a router with VoIP functionality, which the TPG-supplied modem does not have, hence I will continue to use my Billion 7800VDPX. I was however hoping to use the Archer C1200 as an access point / range extender.
Hardware version: Archer C1200 v1.0
Firmware version: 1.00 Build 20180208 re. 59779 (EU)
In its current state, it doesn't appear to have an AP mode. However when checking on the firmware page there does seem to be a US firmware version that allows AP mode.
I got the "unable to update" message when trying both US and EU newer firmware versions.
How do I request TPG to unlock this firmware, if at all possible, since I don't need it as my primary NBN router?
You can use LAN to LAN Configuration.
This way the DHCP task still goes to your Billion 7800VDPX
Just configure the TP-Link and give it an IP address within the subnet of your Billion 7800VDPX but out of it's DHCP range and disable the DHCP server function of the TP-Link C1200
Configure the DHCP range of the Billion to start at 192.168.1.3 and ends with 192.168.1.253.
Then give the TP-Link C1200 an IP address of 192.168.1.2 or 192.168.1.254
Thanks, took your info on board, and also googled "LAN to LAN router configuration" and found more info which confirmed everything for me... surprising how straight forward and easy to remember this process is.