said by EG:FWIW, might they mean the Arris *Dory* gateway rather than "Dongle" ?
Perhaps so; I just did not equate a small external computer plug in module with a small fishing boat (silly me).
If that is indeed the case, then the OP is probably not going to be able to setup the DNS servers used by the gateway because Comcast has not provided that functionality to the customer. I do not have access to one of those boxes at the present time, but I do know that functionality was not originally available in those boxes the last time that I looked at one.