You could check a setting on your iPhone to see if there are any Carrier updates to be installed using the steps in the Apple Support article link below.
Update your carrier settings on your iPhone or iPad - Apple Support
If that does not resolve the issue have you tried a force restart of your iPhone ? if not follow the instructions below (a force restart will not delete any data)
- On an iPhone 7 or iPhone 7 Plus: Press and hold both the Sleep/Wake and Volume Down buttons for at least ten seconds, until you see the Apple logo.
- On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo.
Allow you iPhone to reboot then test to see if the cellular network issue has been resolved.