There's no way to pay a bill online?

I cancelled my T-mobile service and they sent me a bill. I have no access to my past online bills now that I am not a customer. I received a notice to pay the bill in the mail. When I go to t-mobile.com/guestpay as the letter instructs and enter my account number, it says “We are unable to accept guest payments from the phone number provided at this time. To make a payment, please log in.” But when I try to log in, it asks me to link my phone number and once I enter it, it just stays on the same screen. When I called T-Mobile, they said I would have to go into a store to pay. Something seems broken...