Hello Shamil Appa,
It sounds like you've done a bunch of troubleshooting for this issue, but your cellular connection to your provider is getting spotty. You started out with a great set of troubleshooting already, and it looks like there are a few more steps to take yet from the following article:
If you can't connect to a cellular network or cellular data
- Check for a Carrier Settings Update.
- Try another location. If possible, see if other devices are able to connect to your carrier's network in that location.
- If you still can't connect to a cellular network, and have confirmed with your carrier that there isn't an issue with your account or network, try restoring your device.
If none of those work either, then you may need to seek service on your phone:
iPhone Other Repair Options - Apple Support
Thank you for using Apple Support Communities.
Regards,
Sterling