OK, I've solved this one myself. In November 2021 TPG decoupled my mobile account from my main TPG account. It seems the account details from the main account, including payment options and password, were inherited by the mobile account in the transition process. From this point on the details for these accounts would need to be updated independently. I hadn't given it any attention until a few weeks ago when my credit card expired and I received a message from TPG regarding failed payments for the mobile plan. I tried to update the payment details using 'Click here to manage your other services'. Initially, I could access the mobile dashboard, but all the fields were locked with a 'payment processing' message. A day later I was completely shut out and received the same message as the OP. I updated my billing details by calling 131423. Two weeks later I was still locked out of the mobile dashboard.
I figure the mobile account was using an old password imported from the main account in 2021. Never noticed before because the 'Click here to manage your other services' from the main TPG account always worked. Went to TPG's password reset page and changed the mobile account password. The 'other services' button is still broken, but I can now log in directly to the mobile account from TPG's main login page.
