My phone is showing no sim why
Why is my iphone 11 pro max showing no sim after I have rebooted it many times
iPhone 7 Plus, iOS 14
Try to re-seat the SIM (if this is a physical SIM) by removing it and popping it back in, and then turn it off and on again.
If it is an eSIM (and you are sure that there is cell coverage in your vicinity), then best to contact your mobile service provider.