Give it dappled shade, or morning sun and afternoon shade, and make sure not to over-water it. Natal plums are tough, tough trees and will take a lot of abuse. They can drop all of their leaves and still come back. Don't let the pot get too hot in the sun... you can wrap the pot in aluminum foil to reflect the sun's rays.
Inisde of houses may look sunny and well lit to us... but we have eyes that adjust to dimness. The trees may not get enough light in a house, even one that looks quite bright to us. Air conditioning also dries the leaves out quickly.
If leaves have formed in one type of lighting condition, then can't cope with a change (for instance, it possibly was raised in a greenhouse and then went into your home) they may drop off and the tree will make new leaves. But if those leaves form inside the house, and are then put outside, they may be too tender to adapt. So the best thing to do is to find the most promising spot for the tree, and then leave it there. Trees don't have legs, for a reason.
Move it only if the position of the sun changes so much that you need to move the tree slightly.
Good luck!
Joanie