Cellular issue

Try to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

Contact your carrier 

Issues with your carrier or account might affect your service. Contact your carrier to:

• Verify that your account is active and in good standing.
• Make sure that there are no outages in your area.
• Check that your device isn't blocked from receiving cellular services and is set up with the right data plan.

If your carrier confirms that there's no issue with your account or network, but you still can't connect to a cellular network, contact Apple Support.

See also these Apple articles below:

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)