Apple support gave some steps you can do "If your iPhone or iPad shows No Service or Searching", but depending on what we experienced and the tests we made, those steps won't help much.
But this problem is happening with iOS 10.2.1 which is the last update from Apple
But we tested this for more than 10 devices with the same network problem and
we got no result.
- Go to Settings -> Carriers and turn off automatic carrier selection.
Your iPhonemay stop saying No Service if you manually choose which
cellular network toconnect to.
- Go to Settings -> Cellular, and make sure Cellular Data is on. If it is,
try turning it off and back on again.
- Toggle Airplane mode:
- Swipe up the bottom of any screen and the control center will appear.
- Tap the Airplane sign on the top-left corner or control center screen to turn it on.
- Wait for one minute, then turn it off.
"Sorry to tell you that we tried all these but the problem remained".
Of caurse you know how to do that, but it didn't work for us.
It’s easy to understand why an iPhone might say “No Service” if it’s smashed,
but water damage can be invisible and insidious. If your iPhone started to
say "No Service" after it got wet, there’s a good chance that wate damage is
causing the problem
* What if there is no water damage ?!!.
what we recommend for you :
1- Wait until next software update from Apple.
2- Do a hardware diagnostic.
SFS Software provides full diagnostic guide that can take you step by step to the solution. Download SFS