Unable to get 2K resolution on 4K display: mac

Hi guys,

I pull my hair out with this and wonder if anyone can help.

I have a MacBook Pro (Retina, 13 inch, late 2013) and a 4K monitor ACER CB280HK.

I want 2K (2048 × 1080) resolution on my screen, but the only options are these.

I've tried using SwitchResX to get 2048 × 1080, but it's really notched.

I don't understand the specifications that say I can use;

Dual monitor and video mirror: At the same time supports full native resolution on the built-in monitor and up to 2560 with 1600 pixels on up to two external displays, both on millions of colors

I'm connected via thunderbolt ( Mini display port)

Does anyone have any ideas why this might not work? My Windows machine shows 2K just fine on this screen!

Thanks everyone!