It doesn't help that you've got the two routers backwards. The default LAN range on the Buffalo isn't relevant here and changing it did exactly what I said it would.
You don't have to change any DHCP setting. Either use the Netgear as a modem only and use the Buffalo's PPPoE client, or do what I suggested before - add the IP the Netgear gives to the Buffalo's WAN interface to the Netgear's DMZ and you can essentially forget the Netgear exists.
The easier option would be to connect the Buffalo and the Netgear by their switch ports, disable DHCP on the Buffalo and forget it's a router at all.
You don't have to change any DHCP setting. Either use the Netgear as a modem only and use the Buffalo's PPPoE client, or do what I suggested before - add the IP the Netgear gives to the Buffalo's WAN interface to the Netgear's DMZ and you can essentially forget the Netgear exists.
The easier option would be to connect the Buffalo and the Netgear by their switch ports, disable DHCP on the Buffalo and forget it's a router at all.