Interesting setup.
My guess is, your WGA54G is not giving the DIR-825 an IP address through DHCP, so you have to set the WAN IP (Internet setup) to manual & configure the correct setting.
I'm still picking away at this problem - it's important now because we're in an RV resort where I can get WiFi through my WGA54g but my wife often can't directly connecting her notebook. NOT a good situation

It takes several minutes but eventually the DIR-825 gets what appears to me to be correct WAN addresses via DHCP. It gets an IP address like 10.10.10.71 and DNS server addresses like 65.32.1.65. The LAN address range is 192.168.0.10-20.
I assume the router can map my LAN addresses to the WAN address - I mean, that's what it does, right? Or maybe I should change my LAN address to 10.10.10.X???
Cheers, Joe