Sim not supported

I have recently purchased a new SIM card that is different to the original one that was in the phone. When I have gone to put the new sim into the phone it says that the sim I’m not supported. Does anyone know how to fix this? It is rather frustrating I have tried resetting my whole iPhone upon being told it should have unlocked itself from a set carrier as I have had the phone for 3 years now. It hasn’t worked and I don’t know what else to try.