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What lights are lit on the NBN connection box and the wifi router?
Do you have a NBN connection box plugged into the wall socket? What lights are on?
Check that username and password is set in router.
In the article is a description of how to do this and the cabling between router and nbn box.
You'll have to use mobile data.
Here's an article on it.
Forgot your TPG account password?
Then, wait a while (5 minutes) and check you can access your mail through web Post Office.
Login to your wifi router and change the password in the internet section (PPPoE).
That should allow the router to connect to the internet.
Change your password in your email client.