Hi edmek! If you are trying to import the tilesets, I suggest you to use the single files, that way you won't have to slice things up!
Anyways, there are a bunch of Unity devs who managed to do it in our Discord Server, if you need further help!
Viewing post in Modern interiors - RPG Tileset [16X16] comments
Thanks for the reply. It doesnt appear the roombuilder has singles files and that is what I need the most Id say. I don't have a discord unfortunately, any chance you could provide one of the solutions that have worked so far? Sorry for the hassle! I am having this trouble with both interior and exteriors but I am in a point in development where I really need to start building some environments to test how everything works