21:9 works fine in my experience. Either by choosing the resolution specifically, or just leaving the setting to automatic. I can confirm that 2560x1080 and 3440x1440 display perfectly in 3.3.
It sounds like you aren't able to get the right resolution at all, even on the Windows desktop. Are you using a DisplayPort cable? I would make sure you are using that instead of HDMI first.
Doc, this monitor LG - 34WK500-P 34 only has two HDMI ports. What model monitor are you using, if you don't mind me asking.