iphone 6 plus wifi goes on by itself on ios 11

After I updated iPhone 6 plus, my ios to the new ios 11 (and updates after that) my Wi-Fi goes on by itself even when I sut it down. It’s really annoying that the popup for available networks start jumping on my screen many times during the day after I have shut Wi-Fi off.

Does anyone have any suggestions How to fix this? We have really good cell phone plans here in Finland that my Wi-Fi is really bad compared to the mobile internet connection I have.

Please help me!