have you asked Fastweb aDSL about the connection type? it is PPPOE or PPPOA, dynamic ip/dhcp?
by the way, is the dsl light solid on?
you can find more settings under advanced--network---internet--add

Hello, thanks for answering.

I think (I'm optimistic) I solved simply cloning the MAC address of the Fatweb router on the VR2800 and enabling SRA (even if maybe this is not important related to the original problem, but...). After re-registering the modem on-line (a procedure asked from Fastweb) the modem is online: the quality of the line is excellent.

I strangely lost the DynDNS connection to reach the router from outside my LAN, but for the rest everything is working very well by now.