I had similar problems and even tried different providers (EE, Vodafone UK). While I am struggling with connectivity the much smaller Samsung S3 mini of my wife still shows three bars.
For me it turned out to be the contact pads in the phone. What I have not shared is that the problem returns every couple of weeks because of a new build up of oxide. I wish the designers of the FP2 would have thought about this and used proper (fairly sourced) gold plating instead of this silver gunk.
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