Airtel — discontinue service and over charging

Address: interface height , malad west, mumbai -64, 400064

I was using airtel broad band services since 2004.in 2019 i could not get internet for 2/4 months, i was asked to replace the wires though airtel should have fiber line.
So instead of replacing wires i opted for tata boradband, they placed their fiber wire and given speed of 100 mb.
Accordingly i had requested airtel to stop the service and they confirm the service wll be stopped.
Now i have been charged rs 27751/- through credit card as it was auto debit to credit card.
Now had been to their office, i was directed to go to gallery. When i went to gallery, they said it can be done through customer care only.
When i calling customer care the no. I enter is said not valid though i am entering correct no.
So not able to get any help from airtel to get my money back as all other bills were cleared