The thing with P2P on the Internet is everyone is behind NAT routers, so you need a service or your own server which handles connecting clients together since none can actually act as a server from a networking perspective. That isn’t functionality that typically is bundled into 3rd party network API’s from what I’ve seen. Unet handled this via the Unity Multiplayer Service, which was a pay for what you use service technically outside of the Unet API itself.
The one that sticks out in my head is @TwoTen 's MLAPI Relay in conjunction with his MLAPI. I haven’t used it, but the creator is on the forums all the time and knows his stuff.
There’s also the cloud based flavors of Photon to consider if you don’t want to manage your own server for this.