Hi, what do you mean by interconnected? As a cluster with common database of users and apps?
If it is just to make them see each other, then VPN is fine, or even some standard network if they are in a same building. You need to modify the /etc/host file to associate their internal IPs to hostnames on each device wanting to access it though.
I don’t think YunoHost needs internet. You can install applications from local storage.