[NAME]  The system and dock both support DP 1.4, so if the display does as well, then you should be able to run at least 4K 120 Hz over a single connection.  According to the display's user guide, 144 Hz requires you to enable overclocking in the display's on-screen menu.  That said, I remember multiple threads about users not being able to run dual 4K 60 Hz from an XPS 13 9300 connected to a WD19TB, even though that is definitely supposed to work.  So if that still hasn't been fixed (not sure about that since it's been a while), a 4K 120 Hz setup may be similarly problematic.  If you've already updated the Intel Graphics drivers along with the firmware you mentioned, then my only suggestion as a workaround would be to try a USB-C to DP 1.4 cable connected directly to the system just to see if that gives you 4K 120 Hz.  If it does, then you could also try connecting that to the rear USB-C port on the dock near the HDMI output (not to be confused with the downstream TB3 port near the end of the dock where the system cable attaches).  That shouldn't make a difference, but I've had people encounter trouble trying to use DP/HDMI outputs on that dock for display setups that should work that way and then find that it worked fine when they switched to the USB-C/TB3 outputs.  I realize that at the end of the day, this still amounts to a workaround, but if your drivers and firmware are up to date and you've got cables rated for DP 1.4, then this should be working.  So given that it's not, the only thing I can suggest is a workaround.
Hi there.  I have an XPS 9300 (i7-1065g7), WD19TB dock, and Acer XB273K (monitor that supports 4K144Hz), I'm trying to figure out how to get 4K 120Hz on a single monitor. Currently in display settings only up to 4K/60Hz or 1440p/120Hz are available. I'm using DP1.4 cables, with HDR off. Also the monitor has an interesting feature that you can use two DP cables, which I've also tried. I've updated all firmware computer/dock. Any idea why the restriction causing this to not show 4K/120Hz? Thanks!  
4K at 120Hz on XPS 9300 / WD19TB / Acer XB273K(4K144hz)