This is an annoying issue on iOS 11 that has a fix solution on iOS 12.

If you use iOS 11, you will probably feel uncomfortable because it is impossible to completely turn off connections like Wi-Fi or Bluetooth. Therefore, in order to completely turn off these connections, users are forced to access Settings and turn off manually. With iOS 12, although Apple has maintained the setting to temporarily turn off the wireless connection ports in Control Center, users can take advantage of Siri Shortcuts application recently released by Apple to completely turn off Wi-Fi connections. and Bluetooth when not in use to save battery life. As follows. Step 1: Access to the App Store, find and download the Shortcuts app provided for free by Apple. Download completed, start the application from the main screen. Step 2: On the first use, Shortcuts will introduce the features that users can exploit from this application. Click Start. Step 3: The main screen of Shortcuts appears, click “Create shortcut”. Step 4: New creation interface appears. Enter the keyword “Wi-Fi” in the search box and click on the “Set Wi-Fi” result. Step 5: The Set Wi-Fi option is immediately added to the new shortcut list. At this point, switch OFF to this option. Then click the custom icon in the upper right corner. Step 6: Set the name of this new shortcut to “Turn off Wi-Fi” (or optionally set it) and turn ON on the “Display in utility” option. When done, press “Done” to save. Step 7: Exit to the main screen, switch to the Add-ons screen and press “Edit”. Proceed to add “Shortcuts” utility. That’s it, now you just need to click the shortcut to turn off Wi-Fi when you don’t need it. You can do the same with Bluetooth if you want.