Q: Problem on connecting my new iMac late 2015 to Dell U2713H

recently i've purchased a new imac with 10.11.5 and want to connect my dell u2713h as my secondary monitor using dp to mini dp cable.

the result is "no signal detected" from the monitor and swap to power safe mode. however the setting in my imac should detect with the configurable secondary display. blank screen is the result got from my secondary screen.

however when i plug the same monitor and the same cable to my MBA, it works perfectly!

please help.

Chairo