TCP Connexion between Xamarin.Android and SDL_Net

Question

I'm trying to establish a TCP connexion between a C# Xamarin Android app and a C++ app on a Raspberry pi.

On the C# client side, I'm using the TcpClient client from System.Net.Sockets :

// C#
static public bool Connect(string IP, string port)
{
    bool _connected = false;
    try
    {
        client = new TcpClient();
        IPAddress _ip = IPAddress.Parse(IP);
        int _port = int.Parse(port);
        client.Connect(_ip, _port);
        _connected = true;
    }
    catch
    {
        _connected = false;
    }

    return _connected;
}

This is a pretty straight forward use of the TcpClient class as you can see.

On the server side, using SDL_Net (1.2), here is my socket init :

// C++
string mNet::Resolve()
{
    socketSet = SDLNet_AllocSocketSet(2);
    if(socketSet == NULL)
        return "Could not allocate socket set : " + string(SDLNet_GetError());

    if(SDLNet_ResolveHost(&serverIP, NULL, Prefs::NET_PORT) == -1)
        return "Could not resolve server host : " + string(SDLNet_GetError());

    serverSocket = SDLNet_TCP_Open(&serverIP);
    if(!serverSocket)
        return "Could not create server socket : " + string(SDLNet_GetError());

    SDLNet_TCP_AddSocket(socketSet, serverSocket);
    return "OK";
}

and here is a dummy listener for testing :

// C++
void mNet::Update()
{
    int serverSocketActivity = SDLNet_SocketReady(serverSocket);
    if (serverSocketActivity != 0)
    {
        if(!ClientConnected) // new connection
        {
            clientSocket = SDLNet_TCP_Accept(serverSocket);
            SDLNet_TCP_AddSocket(socketSet, clientSocket);
            ClientConnected = true;
            SendToClient("1");
            Logger::Log("Client connected !");
        }
        else // server busy
        {
            SendToClient("0", true);
            Logger::Log("Client refused !");
        }
    }
}

I get no error from the initialization, and when I run a netstat -an | grep tcp command on the raspberry, I can see the port i'm using (1234) opened.

But when I try to connect from the android emulator, it seems that nothing is happening server side (no socket activity). IP & ports are matching.

I'm not really used to networking, so is there something I'm doing wrong here ?

Additional info :

Targeting Android 6

RaspberryPi is running UbunutuMate

Networking C# class : https://github.com/arqtiq/RemotePlayerPi/blob/master/App/Network.cs

Networking C++ class : https://github.com/arqtiq/RemotePlayerPi/blob/master/Server/src/mNet.cpp

Thanks !