Hi the hibiscus bush is probably dying because of the lack of real sunlight, and the same will happen to the tree if you do not get it real sunlight the reason the bush is dying first is because the trees are a lot hardier. You can keep both alive by using two trees one is outside while the other is inside the cage and then keep on switching them but i would only reccomend doing this in the summer (or whenever it gets wamer whatever is first)