You can either contact Fairphone support (calling gives faster results).
Or you can open your phone and have a look … iFixit has a teardown with pictures … you can see the antenna thing in question in Step 6 (the dark long thing that runs along the edge on the left of the phone) … and they disassemble it in Step 11, but that’s not necessary, you just have to check whether the cable is still properly connected on both ends, these are simple plug-in connections …
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