How to sustain TcpListener keep-alive?

public class SecureProxy
{
    private TcpListener listener;
    private HttpClient proxyClient;

    public void Start()
    {
        listener.Server.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
        listener.Server.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, 120);
        listener.Server.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval, 10);
        listener.Server.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, 5);

        listener.Start();
        listener.BeginAcceptTcpClient(ReceiveClientAsync, null);
    }

    void ReceiveClientAsync(IAsyncResult ar)
    {
        var client = listener.EndAcceptTcpClient(ar);
        listener.BeginAcceptTcpClient(ReceiveClientAsync, null);

        using var ns = client.GetStream();
        using var sn = new SslStream(ns, true);

        try
        {
            sn.AuthenticateAsServer(ServerCertificate, ClientCertificateRequired, AllowedProtocols, CheckCertificateRevocation);
        }
        catch
        {
            return; // client refused the self signed cert
        }

        // read the https request and send a https response through sn
        sn.Flush();
    }
}