Not sure if my brightness stopped working from a recent update I did on the system, or it was triggered by the charging cord?? But the brightness on my laptop stopped working a few days ago.
I went on the Dell Support website, downloaded the "Intel HD 620 and Iris Graphics 640" which was all the way down at the bottom on the list of drivers, and installed it. Once installed and restarted, it worked perfectly fine and it adjusted the brightness to where it works properly now.
IF THAT DOESN'T WORK:
One small solution that can help is, if you right click anywhere on your desktop screen, scroll down to "Intel Graphics Settings", click on it and wait for the program to load. Then click "Display" and "Brightness" and that should somewhat help if it's too dim. Otherwise, check the support section or get in contact with a rep.