I'm having the same issue. I linked my home internet service with my mobile to bundle them together for the discount. Ever since doing this I have been unable to access the my account page. Everything else I can log into fine.
When the password gets reset, I type it in and it prompts me at the same page with red text saying immediate password change required. When I try to change the password it says incorrect username or password. I've spent over a week trying to fix this issue now and all the troubleshooting steps get you nowhere.