Cancelled service, lied to about prorated charges and being held for full bill and now late fees

As the title says,

I Cancelled my ATT service over a month ago and the representative 7 days into my billing cycle. I have been an ATT Cellular member for a *very* long time, part of the grandfathered unlimited data plan. I have been on autopay for just as long.

Recently switching to Project Fi to save money, I contacted ATT the day I switched service providers and said that my billing cycle had just recently began and asked if they could prorate, which the representative said yes to. I was told that my adjusted final bill would be in the mail within a months time.

Come a month later, I find that not only am I being charged for the full price for the month, but now I am being charged a late fee on top of that! I feel absolutley taken advantage of and lied to. Calling ATT today wasted an hour of my time. I was passed around to different departments and finally told that there was nobody else that could help me and nothing could be done for me. That if I refused to pay my bill that it would be sent to collections.

Please.

This is absolutely scandellious.

I don't know where to turn. I'm considering contacting an attorney at this point which will humerously enough cost me more than just paying the bill. I'm just sick of ATT getting away with this. They tried to pull something similar when I cancelled my home broadband service years ago and fortunately we were able to resolve that matter. 

Feeling a bit powerless up against this monopoly. How can I resolve this?