Have a look at the troubleshooting in this related article:
If you can't activate your iPhone or iPad (Wi-Fi + Cellular) - Apple Support
If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support
Newsroom Update
Apple and Google deliver support for unwanted tracking alerts in iOS and Android. Learn more >
Apple and Google deliver support for unwanted tracking alerts in iOS and Android. Learn more >
Have a look at the troubleshooting in this related article:
If you can't activate your iPhone or iPad (Wi-Fi + Cellular) - Apple Support
If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support
How to do sim not support