First check the IMEI is not blocked in ur country. If its blocked this guide its not work for u.
This problem normally occurs when u dismount the motherboard for repairs or change the housing.
we must note that in the place where the sim tray is placed, there is a small metal, which when inserting the tray with the sim shifts and breaks contact. When mounting the motherboard, this is a millimeter away from its original position and make the tray does not make contact with the metal.
I leave a picture:
Sorry for my English