What you're asking for is called "NAT Loopback".
It doesn't exist on the friendly firmware of the WZR-HP-G450H. It requires the dd-wrt firmware. I'm in the same boat as you. I think we've been screwed by Buffalo. I'm never going to purchase from them again.
NB. If they ever do get dd-wrt working for the G450H, there's a checkbox to "enable loopback", but the checkbox doesn't work. Instead you have to do this:
Administration > Commands > Firewall Script
insmod ipt_mark
insmod xt_mark
iptables -t mangle -A PREROUTING -i ! `get_wanface` -d `nvram get wan_ipaddr` -j MARK --set-mark 0xd001
iptables -t nat -A POSTROUTING -m mark --mark 0xd001 -j MASQUERADE