It is a 1vs1 turn based game btw and I spawn network objects like this:
// FOR BOTS
GameObject aiObject = InstantiateAIByMode(matchInfo.gameMode, subMode);
AIController aiController = aiObject.GetComponent<AIController>();
#pragma warning disable 618
aiObject.GetComponent<NetworkMatchChecker>().matchId = matchId;
#pragma warning restore 618
NetworkServer.Spawn(aiObject);
// FOR REAL PLAYER
GameObject player = Instantiate(NetworkManager.singleton.playerPrefab);
#pragma warning disable 618
player.GetComponent<NetworkMatchChecker>().matchId = matchId;
#pragma warning restore 618
NetworkServer.AddPlayerForConnection(ownerConn, player);
matchController.player1 = ownerConn.identity;
matchController.player2 = aiObject.GetComponent<NetworkIdentity>();
matchController.aiController = aiController;