cant understand when i was on iOs 5.1.1 i play my sim
Look the OS (like windows or linux) and the BASEBAND is totaly different!
While you can downgrade your OS you can not downgrade this baseband version atm when you was on ios 5.x you as well got an OLDER baseband!
So the solution can be some simcard you put on the top of your sim card (expensive and not always work correctly)