The DSL-302G appears to be a special customisation of this modem for Optus, with DLink forwarding all support requests to Optus.
By default it is configured in a half-bridge mode where the modem is doing the PPPoE login, but NAT is disabled so that only one PC can be connected to it. To connect using this configuration, simply choose the Vigor’s Internet Access Setup > Static or Dynamic IP, Enable the Broadband Access and check that “Obtain an IP address automatically” is selected, then click [OK]. This should provide basic web access, but you may find that some applications do not work correctly – such as: VoIP voice goes one way only, or VPN tunnel creates but no data goes through it.
To get full internet access you should turn the modem into “Full Bridge” mode. This will bypass the firewall and NAT configurations in the DSL-302G, and make it operate as a basic ADSL modem.