Airtel — airtel disconnection request is ignored and being charged

Address:

Bangalore, Karnataka, 560035

I have raised a request for disconnection of broadband on 5th may 2019 and followed up on 29 june 2019 on the disconnection request. Till now, 14 jan 2020, the connection has not been disconnected and i have been charged for 6 months after may.
The reply i have got from customer care when i have called them asking for the reason for not disconnecting and charging is that "though it is their mistake to not to disconnect i have to pay for all the 6 months now then only they will disconnect".
I may have used the broadband by mistake during this period of time due to auto save settings of my laptop/mobile wifi.
Please let me know what should i do.