Hi Ivan,
Thank you for helping me.
The situation has a bit evolved. I can now connect to the FTP server from LAN both in active and passive mode.
My trouble is now accessing it from WAN.
To do so, in a first time, I put the private ip adresse in the dmz of my router box (ADSL modem router) and forwarded port 20 and 21 to this ip in the NAT/PAT setting of this box.
I also configurated the dyndns as explained in doc.
To simulate a call from outside the LAN, I connected from a second PC to the public shared network of my ISP (that provides me an other public adress that the one of my ADSL/modem)
Using Filezilla I get an " waiting time too long, could not establish a connection to the server" (this is translated from French and could have been "timeout"). This is the same in active and passive mode and appears in the first stage (before even exchanging password).
Apparently the dyndns is OK (Filezilla get the actual public adress of the ADSL modem router).
To tell the truth, there is something I don't understand clearly : when I try to enter IP forwarding in the NAS (for example service ftp, protocol TP, external port 20, internal port 20, I get an error. The same for 21.
Do I absolutely need to enter these forwarding rules ?
To be exhaustive here is my FTP server config
Maximum users 10
Idle time 10 mn
Port 21
Passive mode
use the default port range (ticked)
report external Ip in passive (tried both)
client language ISO-8859
Flow control : unlimited
Allow SSL/TLS connections only : no
FXP : enabled