I have an even simpler answer that worked for me!
My problem was the F-cable as well. But I solved it without replacing the F-cable at all. Replacing the F-cable absolutely would have worked, but wasn't necessary.
When I opened up my iPhone 6, I could see part of the F-cable showing through near the top of the phone just left of center. I noticed that there's a small brass/bronze sticker on the F-cable. You can see that rectangular sticker in pictures of the F-cable on the web.
In my case, that sticker had shifted and was no longer covering the brass area underneath fully. I carefully lifted the sticker with the small plastic tool I used to separate the screen from the backing and reseated the sticker. Voila! Full signal and GPS again.
Beauty of this fix is all you need to do is remove the two pentalope screws, suction the glass up, then reseat the sticker. If your sticker is unseated, you don't need to unscrew anything on the inside. Reapply it in the right spot, and close it back up. 5 minute fix.
I suspect this is actually the problem most folks are having because it is hard to see how that cable fails or the connectors fail. But because the sticker seems to sandwich between the top plate and the F-cable, if you drop the phone you can see how the sticker might get pushed out of position. Clearly replacing the F-cable will solve the problem of the sticker having shifted, but if the only problem is the sticker shifted you can save yourself a ton of time and the cost of the cable by simply reseating the sticker.
Hope this helps!