Airtel 4G POSTPAID DONGLE Billing issues

I bought a Airtel 4G POSTPAID DONGLE on last year Dec@2015 from the Airtel agent in Bangalore.

In that i was chosen advance rental plan rs 2999 and paid that money to that agent, also got invoice bill from him.
As per this plan i should not get a bill for two months.

Unfortunately i was getting the bill for those two months around rs 4500.

I had checked with the “Airtel customer care and [email protected] ” were more than 20times regarding this issue but
they has given only answer is “Advance rental plan is not updated in my profile”.So i need to pay as per the plan like that they were informed.

Here my question is , Is it my problem the plan is not updated into my airtel profile ? or who has authority to update my airtel profile ?

Even i have the invoice bill as proof to say , i paid 2999 to airtel agent for the the advance rental plan, but still airtel people are asking me to pay for those months as i paid in advance.

Note:Actually Airtel customer-care support is too bad and they are not ready to solve the problem.

Airtel POSTPAID DONGLE Number : 9148785326