I had an issue with my iPhone with poor WiFi reception and most of the time couldn't connect to WiFi at all. If the WiFi signal from a router was strong, that was the only time I could connect. The Apple store ran their diagnostics and told me there was no fault detected and the option was to pay $429 AUD for a replacement phone so I did some fault finding myself. After reinstalling the firmware through a DFU restore and reinstalling the software to rule out both those contenders it was pretty certain it was a hardware issue.
I replaced the WiFi antenna and also the antenna flex cable while I was in there. The issue is resolved and phone is functioning perfectly. I cannot say which part solved the issue but figured it was worth changing both out at the same time. The repair guide was spot on and super easy to follow, parts supplied from iFIXIT were good quality and did the job. Overall coast was just over $50 AUD including tools and shipping. Thank you!