It's worth trying a direct connection between the display and the system (minus the dock), because it's not clear where you're seeing that the manual for that dock claims 120 Hz support -- everything I see says 60 Hz maximum on video output. [URL]  
I have a Precision 7550 (with a NVidia Quadro T1000 Chip) connected trough a WD19DCS Dock (both USB-C cables) with a single Dell G3223Q monitor connected with a DP cable. The internal display is disabled. I want to use the 120Hz Refresh Rate supported by the monitor. But it is not offered in Windows 10.Since I read in the specs of the dock that 4K 120Hz is supported, I wonder if this is a limitation of the notebook or just a configuration issue? I use the latest drivers for the notebook and the dock.
Precision 7550 4K 120Hz