Restrict an iOS app from using Wi-Fi internet
I'm sorry for it's not possible but you can disabled the app from running.
These are the steps to follow:

Go to General,

Click on Restrictions,

Enter your device password,

Switch off the App you want to disconnect.

I use it to stop Facebook from consuming Data at the background.

FOR NEWER OS VERSION 12.4.6, YOU CAN FIND SIMILAR OPTION IN:

Go to Screen Time,

Click Content &amp; Privacy Restrictions,

Turn on Content &amp; Privacy Restrictions,

Then select what you want to restrict e.g Background App Activities.