Bro, make sure these things
1. your handset is compatible with Airtel-VoLTE
2. Enabled VoLTE switch
3. Restart the device (Restart even if the above two is already done)
If these doesn't work check other Airtel users if they are getting VoLTE in your area or not.
Once VoLTE symbol appears on top bar of android then you'll get data.