I have a Dell U2414H and have the Display Port cable hooked up to my GPU. I fought with this thing for 2 days trying to get it to work because I had an error message saying I had no Display Port Cable. Just by accident I changed the option on the monitor itself to the "Mini Display Port" option. It works now. I don't have the Mini Display port cable plugged in. Just the regular one.
Also, the display looks really dark but bright when I play games on it. Should I mess with the brightness settings? They are already up to 75.