Airtel — slower internet service

I'm airtel postpaid customer. I have availed 499 plan where i get 7gb 4g data. I was rewarded with 10gb 4g data a month for 3 months for installing airtel tv.
Now i'm done with initial 7gb data and the speed of internet went too slow that i cant even open their my airtel app since 24th aug, 2017.
I have contacted their customer service twice and raised a complaint on 29th aug 2017. Complaint number 79035231.
Got a call from airtel saying i need to change data settings on my mobile, that's what they told and i did last 3 times i spoke to them, but it's of no help.
I want this issue to be resolved immediately. And airtel should stop bombarding the customers with free data that they cannot use and slogan "india's fastest network" should be stopped.