Mobile data

Try a Reset All Settings (No Data is Lost)

This resets all your personalized features back to factory defaults

Go to Settings > General > Reset > Reset All Settings

• What this effectively does is to reset some of your Settings to factory default. It’s not a complete removal of settings. It removes all of your preferences for Wi-Fi, Bluetooth, Do No Disturb, Notifications, General, Sounds, Brightness & Wallpaper and Privacy.
• Resetting all settings also reverts all your personalized and customized features like wallpapers and accessibility settings back to factory defaults.
• Performing this action does not impact any of your personal device data, including photos, videos, notes, messages, reminders, emails, calendars, texts, documents, and other files

Is this your corporate iPhone? Do you have any profile installed by your corporate / organisation / enterprise / School Management then please contact the system administrator of your corporate / organisation / enterprise / School Management

If you have installed it on your own --> Install or remove configuration profiles on iPhone - Apple ...

Contact your Carrier (Cellular Service Provider) The agency who issued the SIM to you if they have installed anything through their carrier settings --> Update your carrier settings on your iPhone or iPad - Apple Support

Try --> Check the cellular data usage on your iPhone and iPad ...