LG 28UK850 External 4k monitor to iMac 2015 5K - What cable type/settings?

I have a USB-C to USB-3 cable from the monitor to iMac.

The external screen doesn't come on, and in display settings, no external monitors are found.

When I plug it in, I get a warning on the iMac that reads:

Accessory Needs More Power - Disconnecting other accessories may allow this to work.

The external monitor is plugged into the wall / mains electricity. I am not trying to power it, just trying to use it as a second monitor.

• Can I use the cable I have somehow by altering settings?
• If not, what kind of cable do I need for the 2015 iMac (see port options below)?

LG monitor has USB-C, HDMI, DisplayPort (large/standard size)

iMac 2015 has USB-3 and ThunderBolt 2.