What lights on the NBN modem are on?
Have you received your wifi router from TPG? This connects to the NBN modem. Then your computer and wifi devices connect to the router. It should be preconfigured by TPG.
You need to logon to the router using a cabled computer to set your TPG userid and password.
If that's not possible, it can be done via a wifi device as follows.
Regarding "SSID", this is just the name given to the wifi network in the router. There's probably two: one for 2.4Ghz band and another for 5Ghz band. When the router is turned on, your tablet should be able to scan for one or both networks. Connect to one or the other using the password on the label on the router. At this point, you should be able to logon to the router and set userid and password.