Welcome to the Community!
We've created some articles that will help you with the WiFi issues.
The PPP username and password, it should be your TPG account username and password in order for the modem to authenticate to our server.
You may use a Wired connection in order to access the modem interface and change the settings again. If you don't have any Ethernet cable for Wired connection, you may need to reset the modem.
Note: The modem will go back to its default settings once you've reset it.