if no insight—
Settings>General>Transfer or Reset iPhone>Reset>Reset Network Settings <—————————RESET
If you see SOS, No Service, or Searching on your iPhone or iPad - Apple Support
Manually update your carrier settings on your iPhone or iPad
Manually update your carrier settings on your iPhone or iPad - Apple Support
the basics —if a restart does not solve your problem proceed
Have you shutdown & restarted the iPhone ie power off/on...
Restart your iPhone - Apple Support
Reset or a Force Restart,
Force restart iPhone - Apple Support
Restore from backup, restore as new device.
Restore your iPhone, iPad, or iPod to factory settings using a computer - Apple Support
Put your iPhone in diagnostics mode - identify potential hardware and software issues
How to put your iPhone in diagnostics mode - Apple Support
iPhone Repair and Service - Apple Support
iPhone Repair and Service - Apple Support
Apple Authorized Service Providers and Independent Repair Providers
Repair Provider Verification - Official Apple Support
Check Device Coverage
View Coverage - AppleCare & Warranty