3440x1440 Resolution

Hi can anyone help me out with this:

I have a 21:9 Display with a resolution of 3440x1440. With my x86 linux laptop i"m able to drive it via HDMI@3440x1440p44hz using some arcane xrandr magic. I have not been able to do the same with Coreelec on the odroid n2. I just get a garbled mess instead of an image when i try to manually set that resolution. Since the Odroid n2 supports HDMI2.0 it should (theoretically) be possible to run it at 60hz over HDMI at native resolution. Has anyone got an idea if and how to get that to work?