I also have the same problem before. Wifi cant connect . Official Kitkat from Samsung stock.. But just now I tried flash official MIUI V5 , its based on android 4.2.2. So there is options there to set the Wifi frequency band to 2.4ghz and my Wifi is working again. Connect to wifi immediately. I want to ask anyone, can I install custom rom based on 4.1.2 eventhough I already upgrade to kitkat. ?