Why doesn't my iPhone 4S let me turn on Wi-Fi?

It's a 16GB, running iOS 7.1.2. It's like its greyed out. I'd appreciate it if you could help me.

Restart your iOS device.
Make sure that airplane mode is off by tapping Settings > Airplane Mode.
Reset the network settings by tapping Settings > General > Reset > Reset Network Settings.
This will reset all network settings, including Bluetooth pairing records, Wi-Fi passwords, VPN, and APN settings.
Make sure that your device is using the latest software. To do so, connect your device to your computer and check for updates in iTunes.

If you still can't turn Wi-Fi on, please contact Apple for support and service options. If you can turn Wi-Fi on but are experiencing other issues with Wi-Fi, see how to resolve those Wi-Fi issues.

The solution in this article has been suggested by many youtube uploaders.
http://www.iphoneincanada.ca/how-to/fix-wifi-greyed-out-iphone4s-ios7/

Hope this works for you

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