Create a virtual window to shoot from the real world into your virtual set.
Move the plugin folder "virtualsetwindow" into your unreal plugin folder. On Windows it is normally located under C:\Program Files\Epic Games\UE_x.xx\Engine\Plugins\VirtualProduction.
Open or create your unreal project.
Open "virtualsetwindow Content" in the content browser of your project. Move VSW_Image, VSW_Screen and VSW_Tracker into your scene. Attach VSW_Image and VSW_Tracker to your tracker object. VSW_Screen has to have the same coordinates as your tracker. Give VSW_Tracker the coordinates 0, 0, 0 and VSW_Image the x coordinate of 700 relative to your tracker.
First of all, set up the needed Plug-Ins to use your iPhone or iPad as a virtual camera as listed in the screenshot: