Many people informed me about some devices not working in wifi mode. Those devices have a built-in lock to prevent them to listen broadcast traffic. Android Intercom uses a lot of broadcast because it is the best way to communicate one-to-many.
With the new release those devices will be able participate in talk. Hopefully :)

You need to update all devices to the latest version.

Many thanks to all you informing about bugs, problems and helping me finding problems!