I seem to recall having a similar problem working on my brother-in-law's 624 one time. I'm trying to remember what I did to fix it.. but my memory is failing me.
1) terminate any firewall you're running (disconnect any online connection (cable/dsl) first)
2) make sure you set a static IP address
3) if you use MAC filtering, make sure your first filter covers the wired PC connected to the router.
Hopefully that will get you started.
Wish I could remember what I ran into and how I fixed it, but I recall that I was having trouble entering variables in the router setup pages and it finally locked up and wouldn't let me in to do anything.
I find that once I get a stable configuration and don't mess with things for a long time that I forget some of the finer details of setting up/managing a wireless network.