Charged for an account I cancelled???

I cancelled my service 4/30/19 and ported one of the 4 numbers to Verizon. I spoke with a female at the time and she explained I need to make my account a zero balance and have all devices paid off. I paid off the entire account to a zero balance. I was unaware that we were still being charged until I attempted to put service on a phone that was used on this account. The phone was locked and am being told that my account was suspended due to non payment. When I called into Customer Service and explained that I actually did cancel all service on 4/30 and anything that continued after this I am not responsible for. I was then told by "Rachel" that yes, she does see the mistake made by a prior representative and said it will take 48 hours to credit the account. Fast forward a week and I am still being charged late fees on the account. When I spoke to a new representative today 8/12, they stated "yes I see the Case number created and it will take another 1-2 days. I have no other way of contacting ATT other than getting a new Rep every time. Going to our local store isn't an option as they are 30 miles away and are of NO help.