The selection of which software to use heavily depends on the scope of your project as well as the knowledge of the people maintaining it. As a rule of thumb, always choose the more minimal one over the complicated feature rich one. Most of these software are over engineered anyway.
Anyway, If it’s just communication, use IRC! It’s easy setup an IRC server and install clients on all the devices in the community.
Or if you are aiming for survival, use radio. Like have a proper ham radio setup, it’s much more resilient and reliable (survival stuff is best be kept low tech.)
I’m sorry I have zero knowledge about intranet/extranet stuff. Ig if requires setting up your own like routers and direct wiring between the two communities.