n that case, you make your Buffalo thing into "wifi bridge" mode, and just run a cable from CTU -> Buffalo and disable it's DHCP server etc, and give it some static IP outside the CTU's DHCP range.
Or else, you disable CTU from connecting to asahi-net, disable D HCP server, enable PPPoE forwarding in CTU, then setup Buffalo w/login+password as usual using PPPoE.