Dunno how the reserve league work D, but it'll prob be something to do with finances deciding where teams play, and having to upgrade your team structure to qualify for the top league. That's my guess anyway, hopefully someone will know how it works and answer you properly.
Have a look at this document which is as vague as it is verbose, it alludes to promotion/relegation but doesn’t seem to have a clear description of what actually takes place. It also has wording that could suggest that clubs apply to have their reserves included & May even request the division they want to play in.
What is the situation with the two reserve leagues? We won it last year but there was no promotion/relegation so are these divisions both equal level? If so is that two champions? If that is the case why don’t they play off to decide overall champs?