Chosen Solution
I just replaced my iphone battery and it powers on perfectly but the wifi button is greyed out. Data is working fine as well.
Before ruling out a software issue, try resetting the Network Settings on your iPhone 6 by going to Settings -> General -> Reset -> Reset Network Settings. Resetting the network settings restores your iPhone’s Wi-Fi, Bluetooth, Virtual Private Network, and other network settings to factory defaults. Before you do this, however, make sure you know your Wi-Fi passwords, because ‘Reset Network Settings’ will erase them from your iPhone. After your iPhone reboots, you’ll have to reconnect to your Wi-Fi network by heading to Settings -> Wi-Fi. If the problem still exists after rebooting, you’ve possibly got a hardware issue on your hands A grayed-out Wi-Fi button usually indicates a hardware problem with the Wi-Fi antenna on iPhone. If you’re under warranty, by all means, go through Apple!
