Why is my iPhone 7 Cellular network saying "No Service or Searching" after update to iOS 15.8.3?

There was an inherent issue with some of the iPhone 7's. The hardware would fail leaving the phone without cell service. Apple acknowledged this and repaired it for free for many years. However, that warranty repair expired aa few years back.

If your iPhone 7 no longer works with cellular, you will have to purchase a newer phone.

You might be able to take it to the Genius Bar to have it repaired, but I'm not sure if they still have the parts to fix it and if they did, it probably wouldn't be worth the price of repair.

Genius Bar Reservation and Apple Support Options - Apple

Tap Settings > General > About and check to see if an update from your carrier might be there.

If not, you will need to contact your carrier.......(temporarily using another phone)......to ask them to troubleshoot the connection from their network to your phone.