Extra charge after cancellation

Hi there, I have been using Xfinity for almost three years and recently cancelled the internet service due to the increased rate. However, I got an extra statement that saying I still own one month service fees. I have contacted Xfinity multiple times and spent many hours to address this issue. But nothing happened!! Now Xfinity has sold my debit to Sunrise, this is really frustrating!!! Can someone from Xfinity help me to solve this issue? If anyone else is also facing the same issue, please contact me so that we can work together to push the change!