Mobile fraud and over charging

I was moving from one home to another home. I called Xfinity to change the address. They sold a mobile bundle saying that there is a free phone offer and the line free for 1 year with $100 cashback. 

I didn't check the bill for 10 months, They were continuously charging me for the device and over charging for data for which I already had the data plan. I called billing department and asked them to listen to the conversation. They were saying that even if the conversation was as per what I said. I still need to pay for the mobile. They were shaming me for believing in the Xfinity agent.

I lost a lot of money. I completely lost trust in Xfinity. I hate the dubious plans of Xfinity.

Can someone tell me how can I stop being robbed?