I have a weird problem, and the answers I'm seeing wouldn't apply to my setup.
I've got a Linksys E1550 with TomatoUSB (serving a USB HDD) running as an access point, behind a Linksys WRT310Nv2, which handles all the routing/DHCP/NAT work.
My problem is, I can't (or shouldn't, as it dosn't belong to me) put TomatoUSB on the main router, and I want to use the E1550 for DDNS updates, and the external IP lookup keeps failing because it doesn't have anything connected to the WAN. (I also want to avoid swapping the two, as the WRT310N is Gigabit, whereas the E1550 is Megabit, and there's a fair amount of internal network traffic)
Suggestions have included actually using it as a router, and setting up a new subnet, which I'd like to avoid, as it's extremely useful having no firewall between this router and the rest of this little network.