NetworkObject doesn't get instantiated correctly when spawning as Client

So I've got a Component (ArchitectureManager), which I attached to the GameObject with the NetworkManager
When I'm connecting as a Host, everything seems to work fine, but when I'm connecting as a Client, there are several problems.
For debugging purposes, I build the scene and run it as a host. Then I click Play inside the Unity Editor and join as a Client.
After joining as a client, which seems to be all fine, when I'm looking and the console output, the IsLocalPlayer-Attribute for example from the NetworkOject for the client gameObject is false and when I inspect the clients gameobject in the editor, the NetworkObject component doesn't seem to get instantiated correctly, because there aren't the information, which I can see on the hosts gameobject, like IsLocalPlayer, IsOwner etc.
I attach two screenshots from the editor, while running the scene as a client and inspecting the gameobjects of the host and the client.

However I can see the already joined host player and see him moving around from the clients perspective view, so this way it seems to work.
But from the hosts perspective, I don't see the clients prefab at all, it's just not there in the scene.
Could this be related to the problem with the clients NetworkObject-Instance?

Here is, how I handle the spawning.

Code (CSharp):

1.  
2. class ArchitectureManager : NetworkBehaviour
3. void Start()
4.     {
5.         switch (SceneParameters.ArchitekturModus)
6.         {
7.             case "Host":
8.                 NetworkManager.Singleton.StartHost();
9.                 //spawn 3D Prefab
10.                 spawnedObject = Instantiate(ThreeDimPrefab, new Vector3(32, 2.3f, -10), Quaternion.identity).gameObject;
11.                 spawnedObject.GetComponent<NetworkObject>().SpawnAsPlayerObject(NetworkManager.Singleton.LocalClientId);
12.                 break;
13.             case "Client":
14.                 NetworkManager.Singleton.OnClientConnectedCallback += OnClientConnected;
15.                 NetworkManager.Singleton.StartClient();
16.                 break;
17.         }
18.     }
19.  
20. void OnClientConnected(ulong clientId)
21.     {
22.             //spawn 3D Prefab
23.             var tempObject = Instantiate(ThreeDimPrefab, new Vector3(34, 2.3f, -10), Quaternion.identity).gameObject;
24.             tempObject.GetComponent<SPLNetworkObject>().SpawnPlayerServerRpc(clientId);
25.     }
26.  

Code (CSharp):

1. SPLNetworkObject : NetworkBehaviour
2. [ServerRpc(RequireOwnership = false)]
3.     public void SpawnPlayerServerRpc(ulong clientId)
4.     {
5.         //spawn 3D Prefab
6.         this.GetComponent<NetworkObject>().SpawnAsPlayerObject(clientId);
7.     }

Can anybody enlight me? This is my first time making a multiplayer project, I'm totally stuck with this problem.