Erroneous bill - horrible customer service

I need to file a complaint about how my issue was handled (or mishandled) and I still need my bill zeroed out.

At the end of May 2018, my family and I called AT&T to set up internet at our new home in Texas following our move from Virginia.  The sales rep on the phone told us that AT&T did not provide service to our new location in Texas, but told us that we could elect to get a mobile hotspot for UNLIMITED internet use.  He explained the speed of the internet and what we would be charged and we agreed.  He then arranged for the hotspot to be mailed to us and instructed us to turn it on when we're ready to use it.  When we moved to Texas on 1 June 2018 I turned the device on and began to use it as my primary internet source in my new home.  Not long after, I got a text notification saying that we'd gone over our 10 gig limit.  I called to resolve the issue because, again, we were told we'd be getting unlimited internet with this device.  A second salesman told me that he'd correct the plan to make it unlimited service and I was satisfied that the issue was resolved.  Again, soon after we got a text notification that we were over the 10 gig limit.  We called once again, and finally this agent told us that an unlimited plan was not offered with a hotspot device and that the salesman sold us a plan that did not exist.  We agreed to cancel the plan and send the device back, and we were told that we would be fully refunded and no charges would be left.  On July 17th I received a bill dated July 13th stating that I owe $120.88 and $28.72 was past due and that if the bill was not paid it could effect my credit score.  I immediately called customer service after reading this letter and was put on hold for 30 minutes only to be told that I was transferred to the wrong department.  When I was transferred to someone else, that representative told me he'd check on the situation and get back to me in 2 to 3 minutes.  Another 30 minutes approximately passed and another representative picked up, completely new to the situation, so no information had been passed along.  He didn't know how to resolve my issue so I asked for a supervisor.  I waited for 15 or so more minutes for the supervisor, and I could hear the agent trying to convince the supervisor to take my call.  Evidently the supervisor didn't want to talk to me.  Finally she agreed to take my call and was completely dismissive to my situation and refused to resolve my bill after I sat on the phone waiting for nearly an hour and a half total.  She stated she could not waive the "restocking fee" and an overage charge and that we still owe the money.  Not ONCE were we EVER told about either of these charges, nor were we told we owed anything else at all, but this supervisor refused to listen to my situation.  Not only was this completely poor customer service, I'm being told there's nothing they can do about my bill, even though it was completely 100% AT&T's fault.  I'm stuck with the bill for their error and no one is willing to help me.  I did nothing wrong, the salesperson dishonestly sold me something that is not offered, we were dishonestly told that we owed zero money, and we were not notified of any restocking fees or overage charges that we owed.  I was lied to on multiple occasions and treated completely unfairly and I want this issue fixed as soon as possible.