Software updates

Start with this support article→If you see No Service or Searching on your iPhone or iPad - Apple Support

As you have an iPhone 7 also see this, although the offer may have expired based on how long you have had your phone→iPhone 7 Repair Program for "No Service" Issues - Apple Support

See this Apple article:

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

https://support.apple.com/en-us/HT201415

You need a connection to your cellular network to activate the phone after an update. That might be the problem.