This is how I got my LG 29UM57 to work with my MacBook Air 2015.
I realised that my MacBook Air wasn't able to handle the native resolution of the monitor (2560 x 1080) so I found a workaround (SwitchResX) that gets the computer to - somehow - accomodate the resolution of the monitor.
It just works! This video is about how you configure the program handle the custom resolution of your monitor.