SignalR connection wouldn't start inside a sin

I have setup my signalR connection inside a singleton class so I can use the same connection throughout my entire project. The problem is that the connection never starts and the code never executes beyond the line await hubConnection.Start() however when I do this outside the single class, then the connection is initiated in an instant. I wonder what I'm doing wrong. Here's my definition of the singleton class:

public sealed class ProxySubscriber
{
    private static volatile ProxySubscriber instance;
    private static object syncRoot = new Object();
    private IHubProxy chatHubProxy = null;
    private HubConnection hubConnection = null;
    public event Action<string, string> OnConnect;
    private ProxySubscriber()
    {
        if (hubConnection == null) { hubConnection = new HubConnection("http://cforbeginners.com:901"); }
        if (chatHubProxy == null) { chatHubProxy = hubConnection.CreateHubProxy("ChatHub"); }

        chatHubProxy.On<string, string>("onConnected", (id, username) => OnConnect.Invoke(id, username));
    }

    private async Task<string> StartConnection()
    {
        await hubConnection.Start();

        return "Connection started..";
    }

    public async void InvokeConnect()
    {
        await chatHubProxy.Invoke("Connect", "Mujtaba");
    }
    public static async Task<ProxySubscriber> GetInstance()
    {
        if (instance == null)
        {
            lock (syncRoot)
            {
                if (instance == null)
                {
                    instance = new ProxySubscriber();

                }
            }
        }

        await instance.StartConnection();
        return instance;
    }
}

I'm using the singleton class like this:

ProxySubscriber proxySubscriber = ProxySubscriber.GetInstance().Result;
proxySubscriber.OnConnect += proxySubscriber_OnConnect;
proxySubscriber.InvokeConnect();

First thing, don´t make your GetInstance() async. Use it just to create an instance with no more logic in it.

Make StartConnection public and call it from outside, awaiting the call:

await ProxySubscriber.GetInstance().StartConnection();
ProxySubscriber.GetInstance().OnConnect += proxySubscriber_OnConnect;
ProxySubscriber.GetInstance().InvokeConnect();

If you need to run this code from non async methods:

Task.Factory.StartNew(async () => {
    await ProxySubscriber.GetInstance().StartConnection();
    ProxySubscriber.GetInstance().OnConnect += proxySubscriber_OnConnect;
    ProxySubscriber.GetInstance().InvokeConnect();
});

More details:

The name of your class doesn´t look correct to me. It would be better something like SignalRClientHelper.
You don´t need to check for null values on the constructor, as it will be null with no doubt if (hubConnection == null)
Why do you have a "Connect" method on the server? When you do await hubConnection.Start(); you are already connecting. If you just want to asign a username to a connection, you can pass that username in a header before you call Start(): hubConnection.Headers.Add("username", "Mujtaba");
If you want to make this class reliable, you need to handle SignalR disconnections (but that would be a different question)
Whenever you call chatHubProxy.Invoke("whatever"); you may get an exception if the connection is lost at some point, and that will make your application crash. Wrap it in a try/catch statement and handle the exception as you need.
StartConnection should check if there is already a connection going on. So if you call it 100 times, it would connect just once.