Why Wi-Fi is getting turned-on automatically in iOS 11
Source:

Beginning in iOS 11, the system will automatically try and reconnect to known devices using Bluetooth and Wi-Fi at 5am every morning (local time) if they have only been toggled off using the Control Center.

You 'fix' this behaviour by turning both radios off inside of `Settings' (depending on your device locale and language, "Wi-Fi" might also be named "WLAN" as in the pictures below):


Open Settings -&gt; Wi-Fi.
Switch the Wi-Fi radio button to off.