how to restrict an IOS app from using Wi-Fi

You can turn off an app's ability to use cellular data in Settings>Cellular Data.

However you cannot prevent a particular app from using WiFi, only turn WiFi off completely for the whole phone. One exception to this is an app that has the built in ability to disable Wi-Fi for itself.

Axel F.

That is true however not available in the US. You can provide feed back to Apple here Product Feedback - Apple

Axel F.

There’s no available means to block solely Wi-Fi access from an arbitrary iOS app. It is obviously possible to disable Wi-Fi entirely.