I have a Surface Pro (5th Gen) and I was attempting to use LG's 34" Ultra-wide Monitor (34WK500). I thought it would be great to have the ability to split-screen a paper and a browser window at the same time while both of those windows being, in essence, "full screen." Instead, I had an ultra-wide monitor that was only capable of displaying at 1920 x 1080 instead of the 2650 x 1080. I had the monitor set to display at the "original" aspect ratio, so everything would display correctly at the “suggested” resolution via the Surface Pro’s display settings. Nothing higher than 1920 x 1080 is even displayed as greyed out. Here are the lengths I have gone to rectify the issue:
I have swapped cables, swapped ports, as well as changed driver software for the monitor, integrated graphics, and Surface Dock. I have also troubleshot for the Surface Pro as well. I connected my old, dilapidated core i3 Surface Pro 3. Surprisingly, the monitor worked as advertised so long as the monitor was connected directly to the port on the tablet vice the Dock.
I have also researched this issue, ad nauseum, to learn that users of the Surface Book and Surface Pro 4 have had the same issue that I was experiencing. However, they all have the same thing in common: the mini display port to HDMI adapter. I have learned that there are active adapters as well as passive adapters. I am currently utilizing Microsoft's Surface Mini DisplayPort to HDMI 2.0 Adapter, which is listed as an active adapter, and has a max claimed resolution of 3840 x 2160p @60Hz as well as Twisted Veins HDMI cable which is rated at higher resolutions than what is required for the monitor to simply function correctly.
Needless to say, I have thrown in the towel and exchanged the monitor. I now have a gorgeous Samsung curved display which displays at the right resolution right out of the box. If you can get LG’s ultra-wide monitor to work right, go for it! LG’s monitor was beautiful when it worked and showed such promise but alas, it wasn’t to be for me and my Surface Pro.