My iPhone 6 developed this issue a few months ago (on iOS 10 and iOS 11 update didn't fix it). I paid $10 for both a new flex cable and a Wifi/GPS antennae, and had both parts installed for $35 labor.
It went from only seeing 2.4Ghz and no 5Ghz Wifi to seeing both, and GPS worked again - for only a week, unfortunately after I gave it to my brother in another state.
He sent it back, and the flex cable connector on the Amazon part had gone bad. I had my spares installed, and it was working again so I shipped it back. A week later it was only seeing 5Ghz networks and not 2.4Ghz (opposite of before) and GPS was nuts again.
After almost $100 total parts and labor on 2 failed repairs, without seeing it again the shop is saying maybe the baseband CPU is bad on the logic board, and it's not worth switching from the cheap Amazon parts to the $70 OEM parts. If I'd used their parts from the start ($70 each) they would have warrantied the parts and labor, and I would not be out any money if the repair failed.
Apple would have repaired or replaced the phone for about $250 (128GB unlocked 6), and now they wont touch it because a 3rd party was inside of it.