Wifi Automatically Turning ON after iOS 11 Update
WiFi &amp; Bluetooth under iOS 11 will both automatically switch back on at 5am.
The only way to completely disable either is from Settings.

If you want to completely disable Wi-Fi and Bluetooth for all networks and devices, follow these steps:

To turn off Wi-Fi, go to Settings &gt; Wi-Fi and turn off Wi-Fi.

To turn off Bluetooth, go to Settings &gt; Bluetooth and turn off Bluetooth.



Refs:

Apple KB - Use Bluetooth and Wi-Fi in Control Center with iOS 11
Lifehacker - How to (Really) Turn Off Wi-Fi and Bluetooth in iOS 11