Unauthorized charges

I cancelled XFINITY MOBILE service several months ago. However, I still see my credit card has two charges every month. One is for the COMCAST / XFINITY, which is correct. But the other charge of $19.79 is annotated as XFINITY MOBILE. I could not find any billing statement for that in my Xfinity account. I called the customer service multiple time and I could not get in touch with any real agent. Only be directed to Xfinity Assistant, which is useless in this case. Can someone tell me how to get in touch with an agent to resolve this?