RE: inspiron 15 7567 i5 duplicate display working but extend display not working on external monitor
When you switch to extend mode, make sure you specify a resolution that the AOC display can actually support. For some reason I've seen some displays that only actually support 1080p offer options for 2560x1600 in Windows, at least right after that switch to extend mode is made. Obviously the display work if that's selected, but if your display is saying "Input not supported", that could be the issue. So just for testing consistency, try keeping whatever resolution you're using successfully in mirror mode when you switch over to extend mode.