Connecting external display to late 2015 iMac

I recently purchased the late 2015 27" iMac. I want to use my Asus 278q as a 2nd monitor. I bought a Mini DisplayPort (Thunderbolt Port Compatible) to DisplayPort Cable. I cannot get the 2nd display to show up in the Display menu in system preferences, even when I click the "Detect Display" button (by holding down the Option key). I've made sure I'm on the DisplayPort mode on the Asus monitor. Any thoughts?