veebr0
My server is configure as Private gateway an server
Probably better to configure it as server gateway, private mode will stop incoming data so your dyndns IP will/may not update (??)
1.- My primary domain need to be the same that the dyndns domain?
Yes, if you use the default dyndns client that you setup when you run login as admin and run "configure this server"
2.- How can I force the replication between my server and dyndsn?
I assume you mean replication of the domain name
They have to be the same if above method is used
You can have additional domains on your server, add them in the domains panel
You can configure any number of dyndns domains (or other similar supported domains) on your sme, even when they are configured as virtual domains in the Domains panel.
In that case you need to use the ddclient contrib. It will add a server manager panel, and all "dynamic" domains are managed from there.
You can still use the sme servers dyndns client for the "main" domain, but you do not have to, and it is probably easier to manage all domains from the same place in the ddclient server manager panel.
See
http://wiki.contribs.org/Ddclient