When you are using the Portal Window function, you need to add a Clipping Plane to locate where the set needs to be cut and the objects will move to the Augmented part of the set.

Enabling Clipping Plane

To enable the Clipping Plane, first, you need to enable the Clipping Plane from the Project Settings of the project. Go to Edit > Project Settings > Engine > Rendering and make sure that Support global plane for Planar Reflections is enabled as shown below :

Entering Values on Camera Node

Now go to Nodegraph and go to the CAMERA node, activate the ENABLE property to True, and enter the CLIPPING PLANE transform values from the PORTAL WINDOW properties as the Portal Window is the Clipping Plane of this project. Enter the values as shown below:

And when you check the CHANNEL1 on MULTIVIEWER, you will see that the objects are clipping: