If your iPhone or iPad is saying 'No SIM' or 'Invalid SIM,' there might be an issue with your SIM card, or your device is just having trouble "seeing" the SIM card.

1. Restart your iPhone or iPad

2. Go to "Settings"

3. Tap "General"

4. Tap "About"

5. If your device is due for a software update, select "OK" or "Update" to complete the update (as this might fix the 'No SIM' issue)

6. If this doesn't help, carefully remove your SIM card from the SIM tray, and gently replace it. 

If you are still getting the 'No SIM' or 'Invalid SIM' error, please contact Apple Support so they can help.