Airtel — airtel broadband billing

Address:

Sraddha Palmera Apartments, Bangalore, Karnataka, 560103

I had a six months airtel broadband plan. On 17th oct 2019, i had prepaid for the six months (Rs 8997). But on november, i got a bill of rs 1508. And when i mailed about it they told that the plan got changed to monthly plan without even asking me. Then they told they will adjust the amount and will change my plan to a six months plan. But the plans they sent to me they were of lower prices than the amount i have paid. They told me that they will adjust it with the next bill. But again on 9th november, i got a bill of rs 1392 and now even after mailing repeatedly for 3 times there was no reply from their side. I want to know why this amount has been charged? And every time why this type of thing is happening? Its like a harassment. Instead of doing my work i have to spent a lot of time in writing mails and calling customer service and also not getting any response from that side.