U4919DW, DDM, Surface Dock 2, fixed

I had a very similar problem with my Surface Laptop 3, through a Surface Dock 2, connected to a U4919DW, DDM refused to recognize my monitor. Not sure if it would solve your problem, but I was able to fix it by manually updating my display driver via here. You may need a slightly different driver for your particular Surface. I found the right one by identifying my Display Adapter in the Device Manager and just googled for "Intel Iris Plus graphics driver".

Windows update said that I was up to date, but apparently not since that driver update seemed to solve it.

Good luck.