You might want to read this: https://community.tpg.com.au/t5/NBN-Installation/Connecting-ASUS-RT-AC68U-to-NBN/td-p/20269
The above is for connecting your Asus directly to the NBN box, I believe.
To use it as secondary router, try this:
1. Change your Asus RT-AC68U to have a default gateway of 192.168.2.1 (this is to access your modem settings)
2. Try to see if the subnet mask matches the TPG supplied modem (should be 255.255.255.0)
3. Disable DHCP on your Asus modem.
4. Disable UPnP if it is an option.
5. Connect both routers LAN to LAN.