How do technician fees work? Blindsided by $100 fee and visit that did not resolve our issue

We have been having issues with our connection dropping and reached out to xfinity to resolve - they sent a technician to our home who swapped out our gateway and told us that it was a faulty gateway with a poor fan cooling system. We were even advised by the technician to reach out to a manager at a store to be refunded for our monthly bill as the problem was not our fault. So, I was unpleasantly shocked to find a 100 dollar charge to our account for his visit. This was even more unpleasant due to the fact that his solution did not resolve the problem. I figured it was a mistake and scheduled a time for xfinity to call me - and was aghast that they refused to refund the charge and claimed that they could not even explain why it was charged! All this after spending almost an hour on the phone waiting for these nonanswers! When I brought up that the internet connection is still an issue, they offered to schedule the technician to come back, but I don't want to be paying more randomly selected charges and the person on the phone could not promise that wouldn't happen. 

In reading through information on the technician visit online, I am repeatedly seeing claims that the customer should not pay unless it is not xfinity's equipment that is the issue. As the tech claimed the Xfinity modem was the issue, we should not be paying this hundred dollars. How can I go about getting our 100 dollar refunded? Should I go in person to an xfinity store? Has anyone had luck getting this resolved? I am at my wit's end and see no path forward with the chat process or the phone callback process and feel I should cancel if xfinity will just continue to charge us random fees without any explanation or forewarning.