Core Page Preview plugin is not working properly in iOS. In addition to no documentation how to activated it in edit or view mode, various single selection modes and times will not pop up the preview dialog. Double-tapping will open up the Wikilink.
If there is a way, and how to set it up in preferences, it needs to be documented. If not, this is a bug and it’s a core plugin so the expectation is that it should just work.