Configuring a wired router behind a wireless router and AP

I have a Buffalo WBR-G54 wireless router plugged into the optic fiber
cable modem. The wireless router feeds a Buffalo WLA-G54 assess point
some 4 km away, and the access point is also the hub for a home
network of 2 computers using WinXP and W2K. My internet connection
works ok. I am also signed on for the IPphone service my FTTH ISP
offers. They have sent me an NEC brand (I'm in Japan) VoIP router with
1 WAN and 1 LAN port, and the phone line ports. I've tried plugging a
cable from the access point to both the VoIP router's WAN port and LAN
port. I've tried routing this and filtering that in both router's
configuration tables. All in trial and error, and all in vain. I can't
get the IPphone to work. Anyone have some helpful suggestions to
configure this stuff properly? Or, perhaps what I'm trying to do is
impossible.