Hi @Thierry_Meulemans — Thank you for the clarification, it is appreciated.
When troubleshooting these types of issues, particular when there is a very large network involved, the best approach (always) is to use trial and error. Being as there are numerous networking devices involved in your topology there are a number of variables at play here and the only real way to figure out where the issue is occurring will be to take as many “links” out of the “chain of communication” as possible and then add complexity back in. Our CTO Brian wrote a very informative post on what a healthy network configuration could resemble and examples of where we’ve seen user’s get themselves into trouble in the past.
Moving forward, the first thing I would like you to do (if you don’t have the app already) is to please load FING on one or both (if you choose to) of your android devices to confirm that they are still in the same IP range as the core when you are in those sections (i.e Room 3 & 4 ) of your house.
Lastly, another good test would be to take your other “rooms” offline (temporarily of course and to the best of your ability - all things considered obviously 
), disable any other instances of Roon (except on the Android devices + your core), and leave Rooms 3 & 4 in an “active” state. How do the remote devices respond?
-Eric