SIM card not showing

With the phone on, take the SIM out and wipe it, put it back.

Go through the various suggestions in the support article below but if they don't help, and your network carrier can't help, then  contact Apple Support for further investigation.

Support article:

https://support.apple.com/en-us/HT201415

or: contact Apple Support 

This may also assist you. If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support