In Home service charge

In February, I was having issues with connecting to the email server on my Mac desktop which was connected via WiFi. An Apple tech did some diagnostics and said it was an issue with my internet provider. I contacted Xfinity and went through the online diagnostics without resolving the issue and was told that the next step would be to have a Xfinity tech come to the house. NEVER once was I advised that I would be subject to any charge for the visit. I have been a Xfinity/Comcast customer for almost 20 years and have never been charged for a tech visit. The tech came to the house and I explained that I had numerous Apple devices in the home and the Mac desktop was the only one with an issue. I also told him that Apple had advised that it appeared to be an ISP issue. The tech used a wireless device to check signal/connectivity but did not look at my internet equipment (router/modem). The tech's said that it appeared to be a device issue and that I should contact Apple again. I was subsequently charged $100 for this "diagnosis". The tech made no in house or external repairs, nor did he examine my equipment. Perhaps he didn't need to, but I certainly should not be charged for this visit.