How to change resolution of external monitor from main monitor on Macbook Pro?
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.