I'm having the same issue but only when attempting to log in directly to the webmail post office via a browser. Using an email client such as the built-in Mail applications in both Windows and Android i can receive and send emails.
A workaround for you might be to use one of the above or some other email client: https://support.tpg.com.au/email-settings
I'm wondering if there has been some internal concerns at TPG regarding security so they've intentionally disabled webmail access .... Perhaps a mod will weigh in to explain what's going on?
