We are seeing more and more of these in our shop in San Francisco, four in the last 10 days in fact. We've seen them occasionally over the last year but now we are starting to see them more regularly.
In most of these cases the metal casing is bent. When we bend it back (ie. straighten it out) we are able to re-establish service. I never knew if this was a lasting solution because we never heard back from people.
We've always warned people that we don't know if the connection will last. Two customers recently followed up a couple of days after we got it working and told us it wasn't working again.
I have a theory based on similar problems over the years with other models. I don't think you're going to get anywhere changing the flex cable or other antennas. I believe this is an IC chip issue.
My theory is that when the phone casing bends, the circuit board bends. The circuit board bending causes the solder connections for the chip that deals with cellular reception to break.
By straightening the casing and thus, the circuit board the chip makes the connections to the broken solder joints and we are able to temporarily get the phone on a cellular network. But, the damage has already been done and the solder joints are still broken which means they probably aren't going to keep making contact if the phone is dropped or just even used in a day to day manner.
Unless you know how to do a BGA solder repair there is no permanent fix for this issue.