Unresolved billing issue

Ok so recently I switched over to AT&T from T-Mobile back in April 2017. I visited the eastvale ca location. When signing up, due to a promotion at the time a representative offered a buy on get one promo for the iPhone 7+. With the offer and the PROMISED billing amount I agreed to join. Everything was fine until the first bill. AT&T never paid T-Mobile off for us switching over and then when trying to make a payment we were told the representative put our lines on a business account. The cost of our devices and the down payment was suppose to initially cover the first month and partial of the second month. A few months later (dec 2017) our line cancelled. We then visited the Rancho Cucamonga ca location just to have a representative just as confused as us. Stating we owed $620.00 for unpaid service and even if payed, didn’t guarantee our service being reinstated. She claimed we missed the first two months and late fees kept being assessed. Instead of AT&T reaching out stating so, it’s now been nearly 6-7mo and we have to fix an issue they made. Not only is the customer service bad but why would our line we on until nov if they’re claiming we’ve been late since April? Makes no sense at all. With an hour of no resolution the rep got bothered and told us to go back to the eastvale location (like the gentleman who screwed our account up would remember) or call customer care. Our phones have been off for 2 weeks and no response. So disappointed in this service. Why can’t something be adjusted if it was an error on AT&T!?