I have a mid 2010 21" iMac,ATI Radeon HD 5670 graphics card 512mb. Trying to get a 2nd monitor going, Dell P2415Q. It sees the monitor in the System Preferences and I'm using a low resolution 1920x1080p but the monitor keeps say there is no signal.

I have a mid 2010 21" iMac, ATI Radeon HD 5670 graphics card 512mb. Trying to get a 2nd monitor going, a Dell P2415Q 4K. It sees the Dell monitor in the System Preferences and I'm using a low resolution 1920x1080p ( I know it won't run a 4K monitor) but the Dell monitor keeps saying there is no signal from the computer. The Dell monitor screen is black and there is no way (that I have found) to access its menu other than choosing a signal input. I know my old iMac has the old style Displayport but it does see the monitor. Anyone else get this monitor working with a 2010 iMac? My OS is 10.8.5 and I have tried it on this iMac running 10.10 and still the same problem. I have also tried 2 different cables. Thanks in advance.