The problem with using a cellular booster outside is actually a pretty simple one: if you did get it to work, it wouldn’t cover a very big area no matter what. Cellular boosters are designed for indoor use only, and there is really no way to get around that. They rely on the outdoor wall to buffer the signal between the indoor and outdoor parts of the system. You see, if the outdoor antenna can detect the signal from the indoor antenna at all, the booster will automatically cut power to the system until the two parts can’t see each other. That means most likely you would get 5-10 feet of coverage if you got anything at all.
Now, it’s not like there aren’t exceptions. There are always exceptions. I suppose if you put the outdoor antenna up on a 20 foot mast on one side of your property, and you were trying to boost signal on a patio that was actually 10-15 feet down a hill, and you managed to find a waterproof enclosure that wouldn’t block the cell signal so you could put the indoor antenna outdoors, well theoretically it would work. But come on, that’s a one-in-a-million installation.
The real point here is that cell boosters are designed to avoid the signal from the indoor antenna every reaching the outdoor antenna, because if it did that could create a feedback loop that could cause the entire area to lose cell service. FCC rules mean that cell booster makers must take steps to stop that from happening, and those steps make it pretty much impossible to use a cell booster outdoors. Not completely impossible, but pretty much impossible.
If you need good signal outdoors and you use a smartphone that supports Wi-Fi calling, a much better choice is something like this Ubiquiti access point. Running a cable from your router to this device will give you a long-range Wi-Fi network that easily covers most open spaces like pools. It’s designed to work outdoors and can be configured so your guests can’t see the other devices you have at home.