It seems that some options have changed for the firmware version SBG6580-126.96.36.199-GA-00-488-NOSH
Rg passthrough is no longer available
after some googling and try outs I managed to make it work again
here is my step:
(based on the tutorial here:
1. Unplug the cable from your SBG6580
2. Reset to factory defaults (hold down the reset button on the back panel with a pen for 30 seconds)
3. open your browser and go to http://192.168.0.1 and login with the user name "admin" and password "motorola"
4. Got to Wireless -> Primary Network -> Disabled
5. Copy LAN Mac Address from the Basic > Configuration Section, then go to Advanced -> Options -> Passthrough Mac Addresses -> and Add the LAN MAC Address that you copied & saved.
(I also pasted The WAN address on Configuration Section and the "Cable Modem MAC Address" on Status homepage, that's 3 MAC addresses in passthrough list in total)
6. Basic -> DHCP -> No
7. Basic -> Setup -> NAPT Mode -> Disabled
After disabling NAPT mode you cannot connect to 192.168.0.1 anymore, 192.168.100.1 is still available but a lot of options are missing, so make sure you've done all settings before turning off NAPT.
If you messed up you can just reset to factoru default again and 192.168.0.1 will be available again.