according to the manual that I just found on the dell site
I'm not saying that your manual is wrong or says different, but I do know that for your monitor to work from a graphics card it needs to be plugged into that graphics card.
what I suggest is that you unplug the second monitor for now and try just plugging one monitor into the graphics card and seeing what the device manager says and then come back to us, and I'll go find a moderator and request he reads through the thread and takes it over as you don't seem to happy with my advice.