Wrongful debt collection and report

This is the most livid I have ever been. I feel cheated and victimized by this Xfinity company. This is the internet service I used for the longest time, was on auto payment till I relocated and they don't serve my new area. I cancelled the service since August of last year, but kept getting phone calls about overdue bills. After several calls to difficult to reach agents, I settled to pay $90 for service I never used and have them cancel my service. I kept getting texts still about unreturned equipment ( I bought my own modem) and bills still. I called Xfinity again and was told my the agents to ignore the messages, it is just a system glitch. 4 months down the line, I have a derogatory report on my credit score report. I've used the past 2 days calling different agents, they keep transferring and creating tickets that yield nothing. This is the most corrupt and unreliable company I have ever dealt with. i have never had a derogatory dark on my credit before or a late payment. This company is trying to ruin my hard work and I am so disappointed to think I just recommended this service for my family members in another state.