Tata Photon+ Wifi Broadband — Low speed and excess billing

I subscribed for Tata Photon+ Wifi Broadband service in May. They charged me about Rs. 4000 for the USB modem and agreed to give me 2 months "Unlimited Usage" service for free. However, at the end of 2 months they charged me for usage insinuating that I had exceeded their "Unlimited Usage". According to them "Unlimited Usage" is limited to 10GB. I checked my usage meter, provided by them, and found that I had not exceeded 10GB. However, they insist that the meter at their end shows exceeded usage. I have the option of discontinuing their service. But, I am stuck with their Rs. 4000 USB modem which they are refusing to take back. The whole thing smacks of a scam to cheat customers of their hard earned money.