Over billing small business account

I manage 2 Hallmark stores.  We switched to Att before I got here about a year and a half ago.  We were alerted prices would be going up exponentially unless we switched services.  My boss, called, discussed Fiber etc.  she thought her call was for both stores.  It was honored for one store.  She set up autopay and didn’t realize we were being charged $1200+.   Per month.  We called the sales specialist for fiber in the area.  The owner agreed to Fiber and was told the bills would be going down and they could probably credit back since we could establish we tried earlier.  Yet, it’s been 2 months and we still do not have lower bills.  No one seems to want to help.  What is the next step?!  They are installing fiber as we speak.