Airtel — unnecessary postpaid bill generated

It's regarding my number [protected]. I had got it ported from reliance to airtel 2 months back. At the time of porting, i wasn't informed or asked about whether its a prepaid or postpaid. I was by default subscribed to postpaid billing with an arbitrary high bill of 300 per month.
One more problem with this sim was that i wasn't getting any otp and hence wasn't able to use the my airtel app to check all this information. Neither i got any sms regarding the same. I had raised the complaint to airtel number of times regarding this issue but they did nothing to resolve it.
In the end, when i got fed up with their service and called them to terminate it, they are asking me to pay 607 rupees!
Firstly, i didn't have any information about what i am being billed for so long.
Secondly, it had such a nasty otp problem that my aritel app wasn't working.
I don't want to pay such a huge amount for this [censored] service.