Thanks for contacting the TPG Community, on your modem you should use the Phone1 port (Grey Port) at the rear of the modem.
I have attached a link that may help you.
Good luck - Stay Healthy
You should just need to to connect your phone into the Phone1 port
Make sure your phone is charged and you should have a green light on at the front of the modem where it says 'Phone'
You should not need to configure anything. If that fails power off everything (TPG Modem and NBN box) wait 2 minutes and turn the power back on. After a minute or two it should work.