On iOS devices, when you change the resolution using Screen.SetResolution, the screen will flash and sometimes a short black screen will appear.

[Vortexboy]

This is how I use it.

Screen.SetResolution(w, h, true, 60);

[darkStar27]

Try using

Screen.SetResolution(w, h, true); 

only. Unity will automatically switch to the highest refresh rate supported.

[Vortexboy]

@NorthStar79 Help Me