Sim not supported on an unlocked iPhone

Bought an unlocked iPhone a few days ago as the old one had been damaged and could no longer be used. Tried to put the old sim in the new phone but it wasn’t supported, contacted O2 and they sent a new one out in case the old one was damaged. This new sim still comes up with ‘sim not supported’ but the phone isn’t associated with any networks and is unlocked. O2 have said that if the new sim is still not being supported it may be a hardware issue?