Airtel — airtel hotspot dongle

Address:

755019

I'm having a airtel hotspot dongle which i have purchased from a authorized salesman of airtel, during purchase it was told to me that 'initially the plan is 6 months advanced rental after completion of 6 month it will converted to monthly plan'. But after several communication from my end airtel didn't convert the plan to monthly basis again the plan renewed to advance rental plan and charging a very high amount than current plans.
After complaining to the nodal officer, the organization convert the plan into monthly plan but additionally the total amount of deactivated advanced rental was added with my account. Now the organization insisting me to pay the total value which was not served by the organization.