Consistent charge for modem

When I initially signed up for Xfinity Internet, the system prompted me that I was eligible for a device upgrade, and the price for using the Xfi modem on the website was $0. Therefore, I chose to rent it, and at that time, the customer service assured me multiple times that this device was free. However, I am being charged a rental fee of $15 every month, which is very unreasonable. The nearest Xfinity store is not convenient for me to visit, so I hope you can help me waive this rental fee.