Well, Unity has their own multiplayer framework, although that seems to be geared more towards real-time games, and not so much for turn-based ones.
You might want to consider a service like Firebase for running a turn-based game. You can use the Realtime Database to store game state, Cloud Functions to perform server-side logic and gameplay, and Cloud Messaging to send notifications to let players know that it’s their turn.
I don’t have a tutorial on this available or anything, but there’s some documentation here that you could check out…