Erlang UDP packet not received

I'm trying to get a client (behind a NAT) to send packet to a dedicated server.

Here's my code :

-module(udp_test).

-export([start_client/3, listen/1, send/4, start_listen/1]).

start_client(Host, Port, Packet) ->
    {ok, Socket} = gen_udp:open(0, [{active, true}, binary]),
    io:format("client opened socket=~p~n",[Socket]),
    spawn(?MODULE, send, [Socket, Host, Port, Packet]).

start_listen(Port) ->
    {ok, Socket} = gen_udp:open(Port, [binary]),
    spawn(?MODULE, listen, [Socket]).

listen(Socket) ->
    inet:setopts(Socket, [{active, once}]),
    receive
    {udp, Socket , Host, Port, Bin} ->
        gen_udp:send(Socket, Host, Port, "Got Message"),
        io:format("server received:~p / ~p~n",[Socket, Bin]),
        listen(Socket)
    end.

send(Socket, Host, Port, Packet) ->
    timer:send_after(1000, tryToSend),
    receive
    tryToSend ->
        io:fwrite("Sending: ~p / to ~p / P: ~p~n", [Packet, Host, Port]),
        Val = gen_udp:send(Socket, Host, Port, Packet),
        io:fwrite("Value: ~p~n", [Val]),
        send(Socket, Host, Port, Packet);
    _ ->
        io:fwrite("???~n")
    end.

on the dedicated server I launch the listen function :

# erl -pa ebin
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [smp:4:4] [async-threads:0] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1> udp_test:listen(4000).

on the client side I launch the sending loop :

$ erl -pa ebin                                                      
Erlang R15B (erts-5.9) [source] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9  (abort with ^G)
1> udp_test:start_client("ip.of.my.server", 4000, "HELLO !!!").
client opened socket=#Port<0.737>
<0.33.0>
Sending: "HELLO !!!" / to "ip.of.my.server" / P: 4000
Value: ok
Sending: "HELLO !!!" / to "ip.of.my.server" / P: 4000
Value: ok
Sending: "HELLO !!!" / to "ip.of.my.server" / P: 4000
Value: ok

Although gen_udp:send from the client returns ok, the server doesn't seems to receive any of these packets, as it should print "server received: "HELLO !!!"" on the console.

anyone would have an idea why this is not working.

EDIT 1 :

• There is no firewall or iptable configured on the dedicated server.

• The connection works fine through TCP between the client and the server, but not UDP.

• When I try to run both server and client on the same device (different erlang node), it does not work either.

EDIT 2 : changed the code for the listen part looping on itself re-creating the Socket each time a message is received... but still does not work.