I think this should help you and all rasbperry pi users (no modification to any text files is necessary), I ran into the same problem, where the default was 640x480 resolution, the monitor was unknown and I ended up using xrandr at first and all the other stuff which didn't work. But I realized that ubuntu mate like raspbian does have raspi-config available so you can do is go into the mate terminal then type
sudo raspi-config
then you can choose advanced settings then Resolution, you should be able to select optimal resolution settings. After rebooting the changes should take affect. Well for me I chose the 1920x1080 and oddly enough when I checked the display settings it wasn't monitor: unknown anymore but my samsung monitor this time was recognized, you can view it here, which allowed me to choose from lots of resolutions so I hope this helps the readers too.