I have a Macbook Pro with the four USB-C / thunderbolt ports, running High Sierra. I have a USB-C to DVI cable. I have tried an old 1680×1050 monitor connected it which works fine. However another 2560×1440 monitor does not; it just doesn't display anything when connected, the same as having no signal. That monitor works fine when connected to a PC at full resolution with a DVI to DVI cable.
I'm pretty sure this is because the resolution is too high for the cable. When I change it to mirror the displays in the System Preferences > Displays > Arrangement settings, I can control the resolution of the external monitor from the main laptop screen and set it to 1920×1080 and it works fine. However, as soon as I set it back to not be mirrored, the setting for the external monitor goes back to being displayed on that monitor at the max resolution where I can't see it.
Maybe I should have got a better cable, but being able to change the resolution for the time being seems like it should be really simple, but I don't seem to be able to do it.
Thanks
Best Answer
Try clicking on 'Gather Windows' button in the bottom right corner. It will bring the resolution choices onto the built-in screen. Then you can choose one that suits. If you hold down option while clicking the 'scaled' button, it will bring up all available resolutions.
I have a third party screen, but Mac picked it up as an Apple screen with unusual resolutions and all I could see were grey streaks. Once I gathered the windows and changed to a suitable resolution it worked fine.