Charged installation fee after being repeatedly told I wouldn't be

The site crashed and I'm having to type this all out again. 

I moved into a building with Xfinity services. Had to call to troubleshoot Internet problems. Was told I needed a tech and would NOT be charged as the issue wasn't on my end. A tech came and spent around 3 hours fixing things outside of the building saying it would benefit everyone in the apartment who used Xfinity so I didn't mind taking the time out of my day. He also said if the phone rep told me I wasn't going to be charged then I wouldn't be. He said my modem was old, replaced it with a battered "newer" one then added a splitter to the wires in my unit. Two days later my Internet stopped working and I couldn't schedule another tech for a week. Went without internet. Eventually an Xfinity rep called and walked me thru how to remove the splitter and it started working again. However, I now have an additional charge of $100 on my account despite being reassured I wouldn't be. How can this be removed? I would have ended my Internet services upon losing ACP credit if I knew to expect lies and additional charges. I can't afford unexpected charges.