Unauthorized change in plan and billing amount

I have a plan for 25 Mb, internet only, very basic. I have not been at home for 10 weeks. My only interaction with Xfinity is paying bills each month. My house is empty and nobody uses the service. This month I got a bill for twice the amount, which corresponds to a plan upgrade I never requested or authorized. 

Of course, I have been trying to use their Chat helper, phone numbers, Facebook, etc... nobody responds. I spend 2 hours a day talking to machines that end up on repeating the billing amount and nothing more.

The bill magically increases and plans change without the user knowing since there is nobody from Xfinity to respond. I am quite sure this is illegal.