I ran into the same issue.
To resolve I went into the RD android app, clicked on three horizontal bars in the top left, clicked Display - clicked 'Match this device' and set Orientation to 'Auto-Adjust'. I then clicked on three horizontal bars in the top left, and then the Home Icon.
After that I could connect, with the viewer showing the full primary screen (I normally have two screens setup on my PC) on my phones's screen.
I could zoom in and out with two fingers, in the same way you would normally zoom in and out on an image with your phone.
Hope this helps.