Do you want to replace the existing router with the XR300?
Connect the xr300 in place of the existing one. Do a reset on xr300 which will make it check what type of connection to use. Use a computer or wifi device to finish the setup. When you login to router, it takes you to automatic internet setup. Needs you TPG username and password. And probably ask you to change the default router password. See the label for admin password and wifi SSID and wifi password.
If you do the manual internet setup, the connection type is PPPoE.
The manual has all this in greater detail.
Which router have you connected to the NBN box?
In your reference to '2', is that in an ip address like 192.168.1.2?
If so, it's probably the ip address assigned to your computer or wifi device.
Both routers can have the ip pool addresses changed.
Change IP Pool Starting Address to 192.168.1.3 on the primary router.
On the secondary router, turn off DHCP function, assign its own address as 192.168.1.2;
set default gateway as 192.168.1.1. Connect each with RJ45 between LAN ports 1 and 1.
Somewhere along the way, one or both will reboot and your computer will get a new ip address.