game objects in front of 2D Background and behind dialog

Hey

I have a canvas, in it a panel with an opaque image. At some point in the game the user can click a button to show a settings dialog.

and there are 3D objects in the game itself. they should appear before the background and behind the dialog when it appears.

How can I achieve that? setting the canvas to overlay makes the gameobjects appear behind the UI. but setting it to camera makes the dialog apear behind the gameObjects

Thank you!