If you get an alert that says “Invalid SIM” or “No SIM”, follow these steps:

Step 1. Make sure your SIM card is activated and functional. Test it on another phone if possible. 

Step 2. Check if your iPhone or iPad is up-to-date with the latest version of iOS.

Step 3. Restart your iPhone or iPad. 

Step 4. Check that your carrier settings are up-to-date. Go to “Settings” > “General” > “About”. If an update is needed, you'll see a prompt to select “OK” or “Update”.

Step 5. Verify your SIM Card is correctly inserted. 

Step 6. Try using another SIM card. If you don't have one, go to your carrier's retail store and ask them to test it with another SIM card. You might need to replace your SIM card.