Thanks for the quick answer

I will contact with your sales people.

I'm not an expert cluster-admin but an user. There is not support staff in our department and... In the kingdom of blind, the one-eyed man is king.

Anyway, I'm confused. Probably I don't understand the differences (if any) between "product key" and "serial number".
You mentioned that "you can provide me the latest dss v5 build". Perfect.
I guess I can make a bootable USB pendrive (as I have made from trial v6)

"However, your current build is so old it doesn't have a product key linked to it"
"...to get a new key issued",
here...: does "key" mean the physical device or some alfa numeric code?
Could I use the same "dongle" that I have and overwrite it with the latest v5 build or it is write-protected?
My actual serial number does not mean anything? is it linked only to the dongle?

Thanks again for your patience and advice