This depends on what type of game you do. Most networking solutions (I’d say) allow for cross platform communication.
As you do mobile, I’d recommend a solution with a dedicated server between the clients (distributing the messages). There are a few options and some of those even have managed hosting.
This thread discussed available packages:
The new toolset by Unity is still a bit work in progress. Running dedicated servers with it, means to also run Unity on a machine, which can be benefit but is also costly.