AT T charging me for service that I never received

I have spoken with at least five different people on the phone about this and have gotten no where.  Now today I got a letter from a collection agency saying I owe AT T money.  I never received service and AT and T continued to take money from my credit card even though I cancelled the service from my previous address.

In June of 2018 I moved to a new address and was told that I could transfer my service.  I said great.  Lets do it.  The technician came out and couldn't make it work.  Said he'd have to drill a hole in the wall.  I didn't get permission from my landlord for that so I cancelled with AT T and went with Xfinity.  I returned all equipment and thought that was that.  Then I checked my credit card statement and found that AT T was still charging me.  I called several times and explained and the automatic payments kept on.  I changed my credit card number and called again.  At least twice if not more I was told that I they would go back and refund me the money on my card.  Never happened.   Not only that, but now I'm told I owe AT and T money when I never even for one day had service in my new place. 

I can't believe that no one can help me with this.  I have tried and tried and I have gotten nowhere.  I'm willing to call it a loss for the automatic payments on my credit card but now ATT saying I owe them money is just wrong and it ruins my credit.   Why can't anyone help me with this?