Airtel — incorrect billing: bill generated before current plans ends

Address:

Hyderabad, Andhra Pradesh, 500090

I am using airtel broadband from 2014. Currently using 6+2 months plan which should end in january 2020. But airtel send me a new bill in the month of november itself for rs.5699.81. When called to customer care, it is clearly told me that profile says it is 6+2, there may a mistake in bill generation and forwarded request to billing department. Billing department closed the request without solving and not even contacting me. One executive called me and raised disconnection request. It is totally frustrating and mental stress for me because of this.