How to resolve 'SIM not supported' error on iPhone?

That means the device is locked to a cellular carrier other than the one you are attempting to use.

Only the carrier it is locked to can unlock it. You will have to contact them and request an unlock.

If you are not a current or former customer in good standing of the carrier it is locked it, it is unlikely they will unlock it for you.

Sounds like you need a new SIM card from your cellular service provider (carrier). You device may need to be reprovisioned for the carrier’s network.