hi, I'm also using MG279Q, I've the same problem.
I set the monitor to 90hz (since the freesync range is 35hz-90hz) and i set a frame cap of 90fps global and freesync on. this works with every game.
When i launch ashes of the singularity the monitor is forced to 144hz, no way to change it in settings, and i get the message that freesync has been disabled.
its a shame because i get around 66 fps which is inside the freesync range.. anyway there's no stuttering.