The NBN test portal is saying they have an issue in your area presently and no testing is available.
Suggest in a couple of hours try powercycle ( off and on) the NBN FTTC NCD( Network Connection device - white box) and then power cycle your modem.
In the meantime if you need to reprogram the modem with your TPG username & password this should help
Watch our short video guide on how to set up your NBN FTTC service. Visit https://link.tpg.com.au/hgzDYj "