Cross-host intercontainer communication trouble

More like they need public IP addresses or a VPN between networks. Just switching the IP subnet won’t help. Each host needs to be able to communicate (on UDP ports 500 and 4500, but you can just consider ping for now) with the registered IP (displayed in the host box in the UI) for every other host for the full overlay network to work.

Right now you have 2 disparate networks, so only hosts in the same network will be able to communicate with each other. It is possible to use host labels to schedule containers such that the ones that need to talk to each other all live in the same network.