WIfi issues are real. I just got a replacement phone yesterday. As I sat at the Genius bar, the associate watched my wifi go in and out on its own. Happens at home, school, hospital, and anywhere else there is wifi I can connect to. Apple gave me a free replacement. When I got home a few hours later, this brand new replacement phone was doing the same thing. I immediately contacted Apple, who run diagnostics and told me there was an issue with Settings, but not one that I could fix or one that I could even control. So now I have to go back to the Apple store for yet another phone.
I was told initially that my (first) phone likely had hardware issues. The associate at the Genius Bar said pit was probably software-related. After I got the replacement and had the same problems, I spoke with a Senior Advisor who said software-related. (On another note, the guy that was waiting to be helped after me said he was having the same issue.)
I will say that everyone I have dealt with his been very nice. However, the constant posts to the generic "wifi fix" I see on this forum is insulting. That answer has a rating of 20% helpful, and that's because it doesn't work. There is an issue with iOS 9. There are far too many people experiencing the exact same problem. I've seen posts suggesting a new router might be in order. For a select few, this may be the case. I even bought a new router, to no avail. It was just a waste of money.
My daughter has a 6s just like mine, except she is still on iOS 8, as is my iPad, and we have no issues with those two devices, nor my husband's Android, or our computers.
This is extremely frustrating. It would be great if Apple would at least admit that there is a problem, be it hardware and/or software related, and then give us a solution. At least let us know you're aware of the problem and are working on it. As it stands right now, it's like all of us with these problems are on our own. There's only so much Apple support can do aside from give out free replacements and hope that it works properly.
I switched to iphone because I like how smoothly my iPad runs. No "force close" issues to deal with as on Android, but looking back, I'd rather have the occasional "force close" than constant wifi issues.
I am praying my problem will be resolved next week, but I'm almost at the point that I don't believe there even is a fix. Maybe the third time will be the charm. Fingers crossed.