Hi Chris104,
Can you access your TG797's user interface and make sure your username and password are in correctly.
If you need help doing this you can follow the instructions listed at the bottom of this page:
http://crowdsupport.telstra.com.au/t5/General-Internet-KB/How-to-Change-the-WiFi-channel-on-the-T-Gateway-TG797-ADSL-NBN/ta-p/392816
You can also check that your username and password are correct by using the following self help tools:
Forgotten Password: https://myacct.telstra.com/forgottenPassword?execution=e1s1
Forgotten Username: https://myacct.bigpond.com/forgottenUsername?execution=e1s1