What is the most reliable fix for repeat "No SIM Card Installed" messages?
What is the most reliable fix for repeat "No SIM Card Installed" message on iPhone 11 Pro Max?
The fix would depend on what is causing the problem. If there's no SIM in the phone, you need to get a SIM. If the SIM is defective, you need to get a new one. If the phone has a hardware problem, you need to get it repaired.
The first thing to do is take the SIM out, wipe it with a dry, lint-free cloth and then carefully replace it. If that doesn't work, contact your carrier to see about getting a new one.