I have to agree that there seems to be some magic involved with getting the SBG6580 to cooperate when you configure it to function as a simple modem. It really should not be as convoluted as it is, but it can be done. I documented what worked with my SBG6580 here:
»Another SGB6580 thread - helpuse as modem w/ own router
Skim down to my post in that thread (it's the last one) and try the steps that are outlined there. There are a few subtle differences from the steps that are shown in the first post at the top of the thread.
As I stated in this write-up my recollection is that it was necessary to power-cycle the modem and probably my Netgear router several times to get the external IP address to be assigned by DHCP to my Netgear router. At any rate, after a mind-numbing hour or so it all worked out fine and has run perfectly since then.
"The dog days are over... The dog days are done..."