Seeing as no one answered this question yet, I'll take a stab at it. For the record, I have never tested the individual WiFi/GPS antenna components to know which does what and to be honest, it seems like every supplier has different names and functions associated with these various elements. I sometimes wonder if anyone really knows what these various flexes and cables really do. Hopefully someone who has played around with this will chime in.
Here are a few links with interesting info, although they don't address the WiFi/5GHz/GPS flexes:
How does the antenna design on the iPhone 6 work?
At any rate, poor signal is usually related to these various antenna components being slightly damaged. I would remove the antenna cover and wifi flex and inspect them under magnification to see if anything is torn, stretched or damaged and replace components as necessary. I would also look at the circular connectors at the end of these flexes and cables to insure they are in good condition. Finally, there is also an interconnect cable down by the dock that connects to the logic board as well as some more interconnects along the back side of the logic board.I would inspect all of these to insure you have good connections.
There are various guides you can follow on this page.