Mac Address issue

I’ve owned a cable modem for 4 years, but generally preferred to rent xfinity modem for convenience, tech support, WiFi coverage etc. recently due to financial constraints I’ve decided to go back to my surfboard modem. Only problem is when I tried the internet wouldn’t let me. Customer service told me someone else had my MAC address. I said that’s impossible I’ve owned this for 4 years and never given it to someone else. I am starting to wonder if xfinity is trying to scam me for the extra $14 per month and my next step is to contact the Better Buisness Bureau. I had the problem escalated to management twice, both times they said they would work on it and call me back later. No one ever did. Is there someone who can help me please? Will provide serial # and MAC address privately if needed.