I'm sorry for necro-posting, but I am having the same problem, and I have the same router as the OP.
After going up and down the Support pages on this site I have made sure I did the following.
I have set it to 802.11B/G mode.
I have Disabled NAT filtering (SPI Firewall), as well as Portscan and DoS protection
I have tried all 11 channels and the routers Auto setting
I have tried connecting with and without WEP security
I'm at my wits end, I've done everything that Nintendo has said needed to be done to get the DS Lite to connect.