Have you tried not specifying networks at all? From my understanding of how Docker Compose works, services can communicate with each other out of the box. Just use the name of the service you want to talk to when requested to provide an IP address or hostname (reference = first paragraph of Networking in Compose).
Regarding what is supported by Resin, check out this link.
EDIT: this sounds related to your other post. It also appears you got everything working, right?