Airtel — excess billing - [protected]

Address:

Bangalore, Karnataka, 560024

I was given a airtel hotspot [protected] on surrendering tata photon and the agency collected 350/- towards installation charges and booked for 399/- plan but to my surprise the bill generated had a highest plan.
Am unable to open the bill as the password is sent to the hotspot number, tried sending mails but no response only getting reminders for bill due, have stopped using the device but now another month bill is also generated.