For clarification, so the problem is that you can only cast the screen, meaning you can only duplicate/mirror your desktop screen on your android viewer app, is that correct? MyAARPMedicare
And you want to extend the screen?
If yes, you can easily switch to extension mode via display settings window or display switch (Windows logo + p hotkey -> Extend)
Thank you very much. I am also looking for the same. Can some one please respond.