Are you connecting one of the AG241 LAN ports to the WAN port of the WRT54G? If so you will find that any computers connecting to it would be on a different subnet to any computers plugged into the AG241.

Try connecting one of the AG241 LAN ports to one of the LAN ports of the WRT54G.
Disable DHCP in the WRT54G and allow the AG241 to act as the DHCP server.
Give the WRT54G an IP Address outside the range of the DHCP.