How to make Ubuntu recognize an unknown external display (so I can adjust its resolution)?
Solution 1:
The monitor cable was loose.
When I removed the cable and firmly plugged it in again, everything suddenly worked.
The monitor was recognized, and now I am using my previously desired resolution:
I feel kind of silly, because I never thought merely touching a loose cable would be of any help.
Solution 2:
I reconnected HDMI cable to a second HDMI port on my monitor and it fixed the problem of "unknown display"