Numbers all ported out but still getting charged

I sold my business and the new owner ported out all numbers in late June.  I was charged again for August and paid the bill, despite not having access to the bill since it's only available on tmobile.com.  My login was disabled.  Calling the support line, I don't have my PIN, in the past, they always texted a code to my phone, but since that phone is no longer on TMobile, this isn't possible. 

So, the phone reps can't access my account.  Was told to go to a store and called again with a phone store rep, but the agent on the phone still couldn't access my account.  After about an hour I left.  I challenged the September charge on my credit card. 

This week, I received a letter indicating that the September bill hasn't been paid, presumably before going to collections.  Not sure why I received the letter, since it was charged to my credit card (and disputed).

Very frustrating that it seems I am still getting billed by TMobile yet no one can access my account to tell me if my account is still active and why I am still getting billed.

How can I get help???