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If the new modem came from TPG, then we've created an article that will guide customers on how to set it up.
For a third party modem, just make sure that the general settings are correct. (https://www.tpg.com.au/support/nbn_setup , https://www.tpg.com.au/support/generalsetting.php)
Feel free to search Community for help with your issue, or contact us and we'd be glad to assist you.