No service
My iPhone 7+ continuously shows no service in the top left corner of my phone even with the SIM card in. The only time it changes is if I take the sim out then it says no sim.
These may help articles might be of use.
If you see No Service or Searching on your iPhone or iPad - Apple Support
https://www.payetteforward.com/my-iphone-says-no-service-heres-the-real-fix/
If you can put the SIM in another device to test that will narrow down where the issue probably lies.