AT&T sold me an HTC One A9 and hasnt given me any of the released software updates

I bought the an HTC One A9 new from AT&T around the end of 2016 at the time I was expecting to get frequent updates as they were released because I was buying a new phone. However AT&T totally dropped the ball and almost a month or two after I bought the phone stopped selling HTC phones. I then never recieved updates some of which have been out now for well over a year.Yet AT&T still charges me for the monthly installment plan of a phone that was outdated the a month or two after I bought it. Overall a really bad experience.