It would help if you provided more description of the problem. 😐
If you can't activate your iPhone - Apple Support - https://support.apple.com/HT201407 - "If your iPhone can't be activated or an alert says the activation server is unavailable or the SIM card is unsupported, learn what to do."
If you get an alert about the SIM card in your iPhone - Apple Support - https://support.apple.com/HT201420