Just in case your monitor’s supported resolution is not properly detected or if you want a custom resolution to be used:

In this example, we will try to add 1368×768, with a refresh rate of 60

Run cvt to get the parameters of that resolution on your monitor (if it is supported):

cvt 1368 768 60

It gives you an output like the following

# 1368x768 59.88 Hz (CVT) hsync: 47.79 kHz; pclk: 85.25 MHz
Modeline "1368x768_60.00" 85.25 1368 1440 1576 1784 768 771 781 798 -hsync +vsync

Copy the text starting from the double quotes till the end of the full output and add it to the xrandr command to create a new mode, as follows:

xrandr --newmode "1368x768_60.00" 85.25 1368 1440 1576 1784 768 771 781 798 -hsync 
+vsync

Finally we add that mode to the existing list, using the line in double quotes in the previous output:

xrandr --addmode eDP-1 "1368x768_60.00"

To apply that resolution via command line:

xrandr --output HDMI-1 --mode 1366x768 --rate 60

Where HDMI-1 is the screen output you want to apply on. To know the names you have available and connected, just run the command:

xrandr --query

In my situation, I have the following ones connected DP-1, eDP-1 while HDMI-1, HDMI-2 and DP-2 as disconnected (not hooked to a screen).

To make that persistent, please follow the first link in the sources section.

Enjoy!

Sources:

• http://ubuntuhandbook.org/index.php/2017/04/custom-screen-resolution-ubuntu-desktop/
• https://askubuntu.com/questions/890839/how-can-i-change-resolution-using-xrandr
• https://wiki.archlinux.org/index.php/xrandr