Hathway — AMC service charge for free device

Hathway provided modem device at the time of installation and was free to use until the device the subscription is kept active. However, they have started levying device charges @ Rs. 18 per month for the past 3-4 months stating AMC as the reason.

This charge is not in the contract executed between us (copy of which was not provided to me), was supposed to be free of charges in any form and was not informed at the time of installation, then under what authority are they levying this charge without information and consent of the consumer?

What is the assurance that they will not levy any other charges like this in future?

I tried several times to contact their customer support but got exhausted with either no response or the same copy-paste response every time.

Kindly guide how to take this case with them and make them revoke these charges.