CharacterController.cs

public class CharacterController : MonoBehaviour 
{
    private ClientInputState inputState;
    private Vector3 velocity = Vector3.zero;
    private int simulationFrame;
    
    private void Start()
    {
        NetworkClient.RegisterHandler<SimulationState>(OnServerSimulationStateReceived); 
    }

    private void Update()
    {
        // Only run this code on the client.
        if(NetworkServer.isActive) return; 

        inputState = new ClientInputState
        {
            keysPressed = Keybinds.PressedThisFrame(),
            keysDown = Keybinds.BeindHeldDown(),
            keysReleased = Keybinds.ReleasedThisFrame()
        };
    }

    private void FixedUpdate()
    {
        // Only run this code on the client.
        if(NetworkServer.isActive) return;
        
        // Set the inputState's simulation frame.
        // NOTE: We don't do this in update, as FixedUpdate is called 
        // in conjunction with the physics time step, and not once per frame.
        inputState.simulationFrame = simulationFrame; 

        // Increase the client's simulation frame.
        ++simulationFrame;
    }

    private void OnServerSimulationStateReceived(NetworkConnection connection, SimulationState message)
    {
        // TODO: process the server's simulation state, comparing it against the  
        // client's cached state for the provided simulationFrame. 
    }

}

I wasn't 100% sure how to ask my question since there was a lot I didn't understand, but thanks for taking the time for that reply, I'll take a look into it!