Have you checked if you need to use a fn+f-key or similar shortcut to activate the external display, maybe it’s just not set to automatically switch to external display.
Are you sure you are using the nouveau driver, check /var/log/Xorg.0.log for any errors and to be sure you are using the correct driver.
This guide explains how to install the nvidia driver in dom0, the guide works I’m using the nvidia driver in dom0, installed using the manual method. You properly want to try this as a last resort, it’s not rocket science but breaking the display driver could leave you unable to use any display.