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Welcome to TPG Community!
Just to clarify, are you trying to login to your NBN modem/router interface? If you are, then the username and password should be "admin".
By the way, your TPG credentials are already saved on the TPG supplied modem/router so it is "plug and play" only. Unless, you are going to use a third party NBN modem/router for your service.
If it's a third party device, then usually the information to login to its interface is printed on their sticker on the bottom or the back of it.
Let us know should you require further assistance.