How are you connecting the system to the monitor?  This is a relatively new system with displayport over USB-C, and a very old monitor with only VGA and HDMI inputs. You will need something like this to make the connection, plus a DVI cable [URL]  
Hello,  I have been trying to connect my old Dell Monitor to my new Dell Laptop but it's not detecting it? I could not find a way to know if they are compatible or the problem with the cables. Anyone have any experience with this? Thank you
Inspiron 14 7420 2-in-1, connect to U2412M