What should I do next?

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Level 1c

I'm trying to upgrade to a TPG NBN connection plan, but when I submit the online

registration form I keep getting a message that seems to indicate that my credit

card has expired. IT HASN"T!

I've tried to resolve this problem for over a week now, and got nowhere!

I need an NBN plan as the old network system in my area will apparently soon be turned off.

I'm a disabled pensioner and my phone and computer is my only connection with the outside world. No phone connection will mean I can't order groceries online and get them delivered, etc. etc.
Has anyone any suggestions as to what I should do next?