The HVAC controls are the single most frustrating part of the Niro, period. I have the same question which is easily answered in older cars with a knob for temperature and fan. As best as I can tell, to get ambient unconditioned air, you must first set the desired temperature to the same as (or within a few degrees) of outside temperature by turning the knob. Setting the temperature will cause the fan to come on at some undesired level, then you can reset that level.
Under some mysterious conditions, the AC will come on without the AC button lit. The one non-mysterious one is running defrost. That may trigger both heat and AC. Full manual controls with dials where one glance shows you the current settings are so much easier!
Not sure what the auto mode even does, but I doubt it makes anything easier. One of these days I will get around to reading the clumsy English in the handbook to see if I can make any more sense of the HVAC.
2018 Kia LX HEV Metal Stream with Advanced Tech