ios 8.1.2 wifi grey on iphone 4s

First of all incase of this situation, most of the time it is a software problem rather than a hardware problem. I think it occurs as some of the versions of iOS(be it new or old) softwares are unable to keep all the necessary hardwares intact and working smooth. Some of the hardware parts may not work due to insufficient amount of heat inside the device which make them working. The process which I'm about to state below should work for both new and old iOS.

I have myself lost my Wi-Fi on iPhone 4s after the iOS 9 update and it was literally for above a month or so. I almost gave up hopes after trying different methods(Putting the iPhone in Freezer a couple of times to Using a Hair Dryer to heat it up) to bring the Wi-Fi back to life.. The Wi-Fi came back after those crazy experiments but only for some 10-15 mins.

So by not wasting anymore time lets go straight into the business:

1.If ur Wi-Fi got greyed-out after a particular update then wait for the new update just after that (in my case it got out on iOS 9.1 so I waited for iOS 9.2) and as soon the update arrives install the update using your Mac/PC

2.Before installing the update, get Backup of all your Files,Photos,Videos,Music,Games,etc.

3.After installing the update just to make sure no files of the older iOS persists in the device Erase your iPhone (Settings>General>Reset>Erase All Contents And Settings)

4.After doing that, plug you iPhone to the charging port make sure it charges you iPhone.

5.Increase your Brightness to Full, Screen Auto-Lock time to Never, Turn on 3G and Mobile Data, Open Some Games in the Background, Finally open the Camera App, and leave your iPhone in this set up for 2 hours.

6.Before doing this make sure your battery is at or above 50%.

7.The main reason to do this is to provide internal heat to the device so that it can reset all its hardware components.

8.After 2 hours or above the iPhone should turn hot and you should notice ⚠ (Exclamation Triangle Warning Sign) at the place of the flash on the Camera App.

9.For reassurance you can also try to light up the Torch from the down menu, it should also have been disabled by then.

10.Now leave all the setup as it is just go to the Settings>General>Reset and Rest Network Settings.

11.As the iPhone resets turn on your Wi-Fi router and get back to your iPhone.

12.Toggle up the down menu and you should get back your Wi-Fi.

13.Now after this for 2-3 days while turning on the Wi-Fi on your iPhone just turn on the brightness a bit towards the max(to maintain the proper working of the Wi-Fi chip) so as the Wi-Fi gets stabilised.

If the above mentioned process didn't solve your problem, then I'm sorry to say but I think you are facing a hardware problem as for so you have to visit an Apple Centre to get things fixed.