The modems that we sent are pre-configured with your TPG username & password.
The default log-in details remain the same which is normally indicated on the sticker at the back of the modem.
You may check these articles for reference:
Please don't hesitate to let us know should you require further assistance.
If you are not able to login using the default username and password, then you need to factory reset the modem/router.
This is the only way as we can't retrieve the new admin username and password.
All of the settings that you've changed before will go back to its default once you've performed the factory reset.