You probably know that UNet is deprecated in Unity 2018.3 and removed (yes) from 2019.1. Unity want to build a new low/high level abstraction for the upcoming entity component system. The main problem is that current productions are fucked, especially if they want to upgrade to Unity 2019+.
I just found and tested Mirror, a clone of UNet, that just works! It's exactly the same API. The doc is the same than UNet. Migrating a project from UNet to Mirror is really easy because you just have to change the using statement UnityEngine.Networking to Mirror.
However Mirror doesn't provide a matchmacking service. If you need that, I suggest to use another solution like Photon.
But if your game works with a local solution, it's just perfect!