This question is something I'm working on.
The Multiplayer plugin, as-is, doesn't work on iOS because that system doesn't support WebRTC.
Meanwhile I'm trying to "modify" the Multiplayer plugin to make it work on iOS through a Cordova plug-in ()
I've had no time to test this. But I think could work.
If you need Multiplayer on iOS working ASAP then go for Photon.