You need to set the RTH height to a value appropriate for the area you are flying in. It needs to be set 10 to 20 feet higher than the tallest obstacle in your flight area. When you activate RTH and your altitude is lower than the set value it will stop its motion and ascend to the RTH altitude you set, then fly a straight path to the takeoff position and land. If it is at or above the RTH altitude setting, it will just fly straight to the takeoff position and land.
If you take off from under overhanging object like a tree, covered patio, or such DO NOT use RTH to land the Mantis Q. Be sure to read the RTH section of the User Manual, not just the Quick Start Guide. It should actually be labeled the Quick Crash Guide because it doesn't give you enough information to safely fly the aircraft.