Welcome to the community!
We do not have a interface of your modem/router page, I would love to help and give it a try.
Can you post a screen shot of the modem/router page on this thread? Also, what type of NBN connection do you have?
To better understand the situation, PM us your account details (Username/Customer ID together with the address on file).
I managed to solve the issue, just for people who might also get stuck with the setup, following are some tips:
Thanks for keeping us in the loop and for the detailed instructions. If I may suggest though, it would be great if you can inform the community the type of NBN connection you're using so everyone can have an idea as to what technology the instructions can be applied to.
As for the authentication, in order for it to work, you need to have the correct TPG username and the port / AVC ID allocated by NBNCo should match our records.