I seems to have overcome a bit of the problem. I have a technicolor tg788vn.

I tested a bunch of configuration : firewall enabled/disabled, upnp, port fordwarding etc...

I have been able to join to a lot of server with this configuration :

• Upnp enabled
• Port forwarding 2302 -> 2309
• Firewall enabled (set to 'Standard')
• Add a rule to the firewall : accept ICMP packet from everywhere to everywhere (LAN/WAN)

I believe only the last rule might be relevant. Still weird though.

I am able to join to some server, ping seems fairly well to a bunch of servers now (I had several to none below 2500 before)

A last sad thing :
I tried to join to a server created by a friend (other people were able to join) but it didn't work for me (ping 2500)
I tried to create a server, nobody could join.

I don't know if the ICMP packet thing is really relevant, I will investigate more later. Maybe the UDP flooding rule was just nice yesterday... (post above)