900m2 large house? I assume that rather means the entire property, otherwise the house would be around 30 x 30 m with one floor. If the house is really that big, especially over several floors, I would take more than the two routers.
Our house has 4 floors with a basement, which is about 300 m". On top of that, we have about 600 m2 of grounds/garden. One of the two Asus routers is directly attached to the UPC modem on the 1st floor. The second one is on the ground floor in the living room, about in the middle of the house. The house is almost 100 years old, without concrete walls, a lot of wood. We have reception everywhere in the house. In the garden we still have reception up to a maximum distance of 10 metres from the house.