The first suggestion to fix Cellar Data problem is to turn your iPhone or iPad off and then start it again or restart it may fix the problem .
- Hold Power button down
- Slide The slider of Power off to the right side .
- Hold Power button to start it back .
Turn Airplane On/Off
- Open Control Center by Slide from down to up
- Tap Airplane button
- and click againt to reconnect.
Check Cellular Data on or off ?
- Tap Setting icon .
- Select Cellular from the list.
- Slide Cellular data on .
Reset Network Setting
- Tap "setting" then "general ".
- Click "Reset"
- Click "Reset Network Setting ".
You can update carrier and iOs to fix Cellular Data .and if no one fix the problem , the you need to make backup and make factory reset after that you can restore backup again .