In order to have "smooth" hard edges using normal map without bevels, chamfers etc. UV islands must be separated along hard edges before baking normal map or if it isn't posible to separate UV islands, than edges needs to be welded, of course mix of those two will do the trick also (assuming that the preparation high poly/low poly is done properly) . Sometimes in order to get desirable look i paint some parts of the normal map by hand. Here is an example of a very low poly mobile game model made entirely of hard edges that use normal map to fake smoothness.
https://ibb.co/2qwNs3m