With the Dell Display Manager dmm.exe there are command-line parameters to allow some remote control.
They are as follows:
ddm.exe /IncControl 10 • to increase brightness
ddm.exe /IncControl 12 • to increase contrast
ddm.exe /DecControl 10 • to decrease brightness
ddm.exe /DecControl 12 • to decrease contrast
The default with no second figure set is 1 or 1% - so in my example I use 5 for 5% like so: