Hi!
To solve your problem, try these solutions:
1) Restart your phone: Sometimes a simple restart can solve many problems. To do so:
Press and release the Volume Up button quickly
Quickly press and release the Volume Down button
Long press the Side Button (power) until the Apple logo appears
2) Update software: make sure you have the latest version of iOS software installed on your phone.
3) Check your network settings: Go to Settings > Mobile and make sure "Cellular Data" and "Enable 4G" options are enabled.
4) Reset network settings: go to Settings > General > Reset > Reset network settings. This will erase all saved network settings, so you’ll need to re-enter Wi-Fi passwords and reconfigure your VPN and APN settings.
If the problem persists, contact Apple Support.
I hope I helped you!