The high frequencies, in the gigahertz range, behave very much like a flashlight beam. Best is unobstructed line of sight. Though, just as with the light, there can be reflections off some objects that will provide some signal reception behind otherwise solid obstructions. The signal can also pass through some things that are less dense and non metallic, though with varying degrees of attenuation. Lower radio frequencies can follow the curvature of the earth, but not the WiFi frequencies.
So the non-technical answer is higher is better than lower, as it maintains line of sight better. Make sure you've set RTH altitude to something that'll work.