How do I lodge a formal complaint

We have been Xfinity customers for over 20 years (we are "Diamond" status--what ever that means). We have been back and forth with Xfinity since August 1. I have recently retired and began with reducing our monthly cost. We originally were paying around $250 a month for services. We were told our services would be reduced to $211 a month--a modest savings. Our next bill was $345. We spent more time (4+ hours and 3 different agents) trying to rectify that situation. We finally got an agreement of $192 and were promised Disney+ (including Hulu and ESPN). We actually were able to access Hulu a week+ ago. I just spent another 2 hours on line with 2 agents and a supervisor and were told we don't have Disney+ and Hulu (yet my husband is on ESPN as I type this).

Xfinity and customer service do not align. In our area, Xfinity is pretty much an "only choice" and boy, do they use that to their advantage. 

I cannot fault the agents... they are only doing what they need to to keep their jobs and provide for their needs. 

I have tons of screenshots and transcripts of the promises made to us.