That’s a very big question. The Digi product manual is the primary source of information - it can be tough going at first, but just about everything you need to know is in there. You might also want to look at the unofficial cookbook and FAQ (see pinned posts) for another angle on things.
You’ve already said that in fact the modules are communicating, so you must have the correct options already set for them to talk to each other. Check the documentation I’ve mentioned, and see whether that leads to more specific questions.