Mobile data

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 ...

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