You will have to remove the back cover, the battery, and the black plastic shield (probably) to check the 3g antenna connection. Its a tiny ball and socket thing on a lead coming in from the band on the non-speaker side. Sometimes it is found to be loose or not well attached. Wiggle it around a bit.
Some times it has been found to be grounding against some metal bit its not supposed to be touching. A bit of nonconducting tape can fix that. Some times the antenna in the band or the wire to it is just bad - can't help you there.
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