Using Dell U3014 monitor with new 27" 5k iMac

I just bought a new 27" iMac with Retina 5k display. I got the model with the Radeon Pro 580 with 8GB of VRAM. I am now trying to use it to also drive an external monitor, my Dell UltraSharp U3014 30" display.

I used to drive this monitor from my MacBook Pro using a DualLink adapter. That adapter plugged into both the USB and Thunderbolt ports of my MacBook Pro, and into both the DVI and USB-B ports of the Dell monitor.

My new iMac, of course, does not have the same Thunderbolt port that my MacBook Pro had, it only has USB-A ports and USB-C (Thunderbolt 3) ports.

I bought a USB-C to HDMI adapter, and tried connecting them both. The iMac recognizes that the U3014 is plugged in, and offers me appropriate resolution options, but the Dell says "no signal" coming from HDMI port and goes to sleep. Notably, the iMac does not offer me refresh rate options, it only says "60Hz" and it's greyed out.

Do I need to get a Thunderbolt 3 to Thunderbolt 2 adapter and use my DualLink adapter, or am I doing something wrong in setting up the iMac to use HDMI?