The purpose of the humidistat on the fan is to turn the fan on when the attic becomes humid to bring in outside air to dilute the moisture with drier outside air. If the outside air humidity is higher than the attic air making the attic humid, then turning on the fan with the humidistat makes no sense. You would be better off with no humidistat as heat from inside the house to the attic and sun falling on the roof warms the attic air making the relative humidity decrease.
A comparative humidistat to compare indoor to outdoor humidity is an expensive laboratory device. You may want to set an outdoor humidistat to sense outdoor air and only permit the fan to come on when the outdoor humidistat senses lower humidity and the indoor humidistat senses high attic humidity.