Can’t activate due to past tenant

I purchased Xfinity internet service  and after much effort of trying to activate my service, was told by an associate that a previous tenant in my building had Xfinity and therefore I can’t activate my modem/router. What am I supposed to do? I’ve been sent a billing statement and I haven’t even been able to use it at all yet, nor can I even be sure the person who had Xfinity here doesn’t still live in the building. I personally don’t and have never had Xfinity internet, so why is this effecting me?