Welcome to the Community!
I checked your account using your community details as a reference, but it seems like the main reason why your modem is having difficulty with authentication is due to an ongoing outage. From what I can tell, you have spoken to our Helpdesk Team earlier who have already advised you of the same.
The outage started yesterday (Wednesday 13 March 2019, 4:00 PM AEDST) while the current Estimated Time of Restoration is Friday 15 March 2019, 7:00 AM AEDST. Please be advised that the ETR is subject to change depending on further assessment. For updates regarding the outage, you can visit this link: http://tpg.com.au/servicestatus.