The trick is getting it across. Without using a character new to the area, whether a transplant from our world or from another region. That's just the classic question of motives and dialog. (Instructing children in a class or other formal education is the same principle.)
But -- a place where you have the Fisher King effect, it really matters what are the lands that are affected by the health and/or virtue and/or even moods of the monarch. Does this effect necessarily correspond to the regions that he has political control? If so, how much control? If not, do you use, say, kingdom to mean those lands where the king's well-being affects the fertility of the land, and realm to indicate where he actually has clout, whether that is a smaller region or a larger region than the kingdom? One can see a conquered island being peeved that they do not enjoy the enchanted prosperity of the mainland -- or of course, mainland dwellers who envy it its even prosperity, or its stout independence -- but whichever way it goes, the natural way to talk about it is about those parts of his realm that are not of kingdom, without defining either realm or kingdom for the benefit of the reading audience.
Some terms, of course, have a secondary meaning built in by the accumulating tropes of fantasy or SF. Very few readers of either would trip over a gate that hurtles you through space or even time. Then again, not including enough for other readers confines the writing audience.