Do you know what video card you have? Check Device Manager, Display adapters and tell us what video card. The Intel one is the integrated video and the second one is your video card. Here's the manual for the monitor. Dell S2421HS Monitor User's Guide Make sure you use the HDMI cable to connect to the monitor and do not use integrated video ports on the computer (if you have a separate video card which most 8700 have.)  The integrated ports  are on the back upper left. The video card connections are at the bottom back of the computer in a horizontal slot. See the illustrations in the XPS 8700 manual xps 8700 Owner's Manual (dell.com)
Can't get new S2421HS Monitor to work on my XPS8700.  Message says I have an unsupported video configuration add in graphics card.  Don't understand this, the only thing we have added to computer was additonal memory, I'm 87 and not too computer savvy, so don't have a clue
S2421HS