My iPhone 4s wifi has been disabled/grayed-out. It seems irreparable as I tried all methods on internet and got it checked on repair shop. Just thinking can I use wifi with iPhone by connecting it to any external wifi adapter or through OTG? Is there any external wifi adapter available in market for iPhone?
This is how you fix this particular problem
A common problem in the iPhone 4S, sometimes the Wi-Fi button will be grayed out and unclickable. This problem seems to be related to thermal shock — the problem may be temporarily fixed by simply putting the phone in the refrigerator for 15 minutes, or under a lamp for 30 minutes.
If this is the case, then the necessary permanent solution is to reflow the Murata SW SS1830010 Wi-Fi chip on the logic board.
Make sure you take it to a professional repair shop. Not a kiosk in a mall that does simple screen swaps. Not every repair shop is going to want to attempt this repair and some may try to rip you off and say that they can do it when they really can't and just take your money. Good luck!
No, you can't an external Wi-Fi-adapter to an iPhone 4S using the Dock-Connector, Bluetooth or any other way.
You may try to re-install iOS 9 before bringing it to repair shop. Most probably it is an issue with the logic board.