As I explain in my books, assuming your video card has a connection to an external monitor (HDMI, S-Video, VGA or DVI), it should be as simple as going to Options/Preferences and, on the Preview Device tab, selecting the Device from the drop-down menu.
You can then activate the view at any time by clicking on the Video Preview on External Monitor button the Preview panel.
My video card does support dual monitors. When I have both monitors connected, then select the Options as you said, when I click the blue button to show full screen on external monitor, the full screen appears on my main monitor rather than the external monitor. But I did find a way. By undocking the preview window and dragging it to the second monitor, I was then able to resize the preview window to full screen. That is a bit of a hassle and seems it shouldn't be necessary, but otherwise, the full screen opens over my editing interface on the main monitor.
You also need to make sure the the proper device is selected. When you select your preview device you will see a 1 or 2 select 2 (1)is your default main monitor