Sim carrier support

It sounds like your phone is “Carrier Locked” to a carrier OTHER than the SIM you are trying to use.

Only the “locking carrier” can release the lock.

How to unlock your iPhone for use with a different carrier - Apple Support

If you are not an active, paying customer of THAT carrier, this might be a difficult proposition.

Do you mean the physical frame that hold the SIM? If that is your issue you must get a SIM that fits the holder, not a holder that fits the SIM. Note, however, that all phones sold in the past 10 years use the same size nanoSIM, so if you can’t get the SIM into the SIM holder frame you are not inserting it correctly into the holder.

If you mean your phone says “SIM unsupported” that has nothing to do with the cellular network provider. That means that the phone is locked to a specific carrier, and you can only use SIM cards issued by that carrier, unless the carrier that the phone is locked to unlocks the phone. ONLY the carrier that has locked the phone can unlock it→How to unlock your iPhone for use with a different carrier - Apple Support

You would need to contact the carrier the iPhone is locked to, and get them to unlock it. See here for details: link-> How to unlock your iPhone for use with a different carrier - Apple Support