Problem with SIM signals

I'm not sure what you're asking about. Terse questions can be difficult.

SIM cards have electrical contacts, and cellular networks have signals.

SIM cards, SIM card electrical contacts:

Remove or switch the SIM card in your iPhone or iPad - Apple Support

If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support

Searching for service, signals:

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

iPhone 7 Repair Program for "No Service" Issues - Apple Support

What is the exact error message that you get?