Night Light not working on external monitor (through DisplayLink dock)

You can use Brightness Controller, a GUI frontend of xrandr, which supports multiple monitors.

sudo add-apt-repository ppa:apandada1/brightness-controller
sudo apt update
sudo apt install brightness-controller

Now you can select your primary and secondary monitor, and change their RGB color temperatures using the sliders. There is an option to save your current settings, which will be automatically loaded the next time you open the software. You can add the software to Startup Applications.

You might have to disable "Night Light" to use Brightness Controller, but it works as a substitute for Night Light. For more details see https://www.omgubuntu.co.uk/2017/05/adjust-external-monitor-brightness-ubuntu

there is a command line software called xrandr, it should be installed already but if not sudo apt install xrandr

If you run it, xrandr will produce all of your monitors and their resolutions they can be accepted.

For example for me:

Screen 0: minimum 8 x 8, current 2560 x 1080, maximum 32767 x 32767
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-0 connected primary 2560x1080+0+0 (normal left inverted right x axis y     axis) 677mm x 290mm
   2560x1080     60.00*+
   1920x1080     60.00    59.94    50.00    29.97  
   1680x1050     59.95  
   1600x900      60.00  
   1280x1024     75.02    60.02  
   1280x720      60.00    59.94    50.00  
   1152x864      75.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x480       59.94  
   640x480       75.00    59.94    59.93  
eDP-1-1 connected (normal left inverted right x axis y axis)
   1920x1080    120.04 +  48.01

I understand that I have two monitors, HDMI-0(my external monitor) and eDP-1-1(my built-in monitor), now stop here :) if you don't have two monitors here, I mean if you missed one of your monitors, install Nvidia driver first, then check if you have xorg.conf on your system, if you don't have this file, make a .sh file and put these in it:

#!/bin/bash
sudo rm -f /etc/X11/xorg.conf
sudo rm -f xorg.conf*
sudo service lightdm stop
sudo service gdm stop
sudo service kdm stop
sudo service lxdm stop
sudo service xdm stop
sudo service wdm stop
sudo Xorg -configure
[ -f xorg.conf* ] && sudo mv xorg.conf* /etc/X11/xorg.conf
sudo dpkg-reconfigure $(dpkg -l | awk '{print $2}' | grep "^xserver" | tr '\n' ' ')
sudo update-initramfs -u

make it executable(sudo chmod 755 ./thisfile.sh) and then run it(./thisfile.sh)

It will create the xorg.conf file (and actually reset it for you). Maybe a restart needs to be apply after running this script to ensure all the things are ok.

Now if all the things be ok, if you now run xrandr again, you have to get your two monitors in the list.

I use the below code to turn my built-in monitor off(the eDP-1-1, you have to change this to yours), and turn my HDMI monitor on with dpi of 120 and best resolution of it(HDMI-0, you have to change this device name to your external monitor name listed in xrandr output)

I also suggest you to learn more about xrandr, it's really one of the best tools in Linux I saw :) it can change your resolution, brightnes of monitor, dpi, panning, scaling and many other things.

xrandr --output eDP-1-1 --off --output HDMI-0 --auto --panning 0x0 --primary --dpi 120

I hope this solves your problem.