You got the wrong dock .  WD19TBS is what you should be looking for  I think you got the type C version
Hi, I have my XPS15 connected to a WD19 dock. 2 Displays are connected to the dock. The first display is a 4K screen connected using DisplayPort. I had the second display is a FullHD screen. I had this one connected using HDMI, but some text was flickering. So I decided to also connect this second display using DisplayPort. The flickering is gone, but now the maximum resolution I can select is 1600 x 900.   Any idea why I can't select FHD for this second DisplayPort connection?    Regards
WD19 dock video resolutions