BILLING ISSUE

Hello,

I had my Xfinity WiFi installed late July/early August. My normal bill is $50 including the install would have made it $150 due the 29th of August. However, I called Xfinity customer service three times, far before August 29th, to set up a payment arrangement for the install so that I could pay $50 in August and $50 in September which would have made each bill $100. However, the total in my billing details never changed and by the end of August, I was still showing due $150. On the third call to set up the payment arrangement, the customer service representative canceled my autopay advising me to pay the $100 after 48 hours of the phone call which actually made my payment late but I was assured it was okay. After 48 hours, the bill still showed $150 due. I called a fourth time, and  the customer service agent told me to go ahead and pay the $100 for August and the next $100 in September. Now I am getting emails and calls saying my bill is late and my service may be cut. I also incurred extra fee because the autopay was cancelled. How can this be reconciled to suit both myself and Xfinity?