Why is my iPhone searching for service

Go through the troubleshooting steps in this support article.

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

1) Connect to Itunes.

2) Backup data on the device.

3) Disable findmyiphone or Sign out of icloud.

4) Reload iOS by pressing restore at next to update at the top most part of the screen.

This should resolve your problem, restore data once problem has been solved.