Unable to Cancel Mobile Plan and Refund not received

To whom it may concern, 

Last year I created an internet account with xfinity and despite not wanting it I was signed up for the mobile add on which I never used. I moved out of an area where xFinity offers services and so I canceled my internet account. I have since started receiving bills for the mobile account. Last month after a second bill I chatted with an agent who agreed to refund the unused service and helped me cancel the account. I never received the refund and on top of that received a bill for the next months services. The card that was connected with my Xfinity account had been disconnected and so the payment didn't go through. I thought that that was okay as I had cancelled my service which I had never used and that would be the end of it. However, now this bill has been sent to a collecting agency and I would really like to get this resolved. 

To summarize I have been paying for a service that I do not use and I have not been able to cancel it or get a refund for it. 

Thank you!