If everything works, put the PCB board back with the dock-connector in first.

Connect all 8 of the flex connectors.

Screw all 11 screws back into place.

Do not close the cover.

While the cover is open, test the 3G function several times to make sure it consistently works. Test again after the screws are tightened.

Loosen some of the screws if the 3G SIM is failing, as this could be the cause of the problem. If needed, insert spacers between the PCB board and case around the SIM slot.

If you do this, the spring contact between the PCB board and the 3G antenna should be widened a little.