ok, so my problem was fixed by manually adding the ISP's DNS IP address to the setup within the router.
I also have a speedstream modem and 520-GC, but connect through AAPT, ISP shouldn't matter in this instance.
If you can't get the IP of the DNS server by calling optus, plug the ethernet cable back into the modem and one laptop, and find it in the web-based modem config (192.168.1.254).
unplug the ethernet cable from your modem/laptop and back into the router. enter the web-config of the router (usually 192.168.1.1), 'Finish' 'save and restart'.
once restarted, you will likely need to restart all computers. if this still doesn't work, command prompt on each computer and type ipconfig /registerdns.
hopefully this will solve it completely.
Nifty