Airtel — 4g network connectivity problem

Address:

New Delhi, Delhi, 110052

When i on data in my phone a small exclamation sign appear on network sign. I don't no why this happening and data is not working. I complained many time in customer care. But there is no any satisfactory solution. They gave me 24-48 hours time to resolve this problem. But after 25-30 days this problem is still there. Airtel service is very bad in these days.