I bought this cable to connect my external monitor to my new iMac as a second display. https://www.amazon.com/gp/product/B071D7XFGQ/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1 The iMac isn't recognizing the display. Any idea why it isn't working? My monitor only has one DisplayPort so I'm on the right input. Should't I just have to go to the Displays Control Panel? I tried both usb-c ports on the iMac. Thanks.