You're out of luck, if none of the route segments is served by Deutsche Bahn (which is often the case for regional routes), because then you'll be unable to buy your ticket using the most common nation-wide app DB Navigator.
https://www.bahn.de/angebot/regio/verbuende
Not all of them, I'm sure, but all of the major ones seem to be covered.
I'm using it every day, because it's better than using the association's own app, which is not saying much. Or I used to, anyway, now I'm doing the 49 EUR thing.