Hello everybody, I've been struggling to connect my macbook pro to my external monitor for hours.. but I finally found a fix that worked for me.
I connected the monitor to the laptop using a USB C adapter and a HDMI cable. What I did was:
After having connected both devices, go to System preferences/Display/Display settings... (bottom left)
Select your external monitor
Change the value of "Use as:" from "extended" to "mirror.."
Change back by clicking "stop mirroing" in the same field
It should bring back the "extended" option and the monitor will now work properly
Hope it helps, cheers