Why is my SIM card not working

1. Make sure that you have an active plan with your wireless carrier. 
2. Restart your iPhone or iPad.
3. Check for a carrier settings update. Go to Settings > General > About. If an update is available, you'll see a prompt to select OK or Update.
4. Remove your iPhone SIM card or iPad SIM card from the SIM card tray and then put the SIM card back. Make sure that the SIM tray closes completely and isn’t loose. If you use a SIM tray from a different model iPhone, iPad, or from another phone manufacturer, it might not fit properly.
5. Try using another SIM card. If you don't have one, go to your carrier's retail store and ask them to test with another SIM card. You might need to replace your SIM card.

If you still see an alert about your SIM card, contact Apple support.

Learn what to do if you see a SIM not supported alert.

Hello,

Please check if your SIM is supported by your device (if your device is not factory unlocked, it will only work for specific carrier/telco SIM). If so, have you tried turning the device off and on? Resetting network settings? And last check, contact your telco/carrier provider to check if your SIM is active (if you don't have another device you can use to test if the SIM is active or not).

Good luck!