Normally, the WAN port of a router should connect to a modem, or a modem/router in full bridge, in order to obtain a WAN IP address. AFAIK the HH5 has no full bridge mode (would be nice if it did, does anyone know?).
The WAN port may do what you require, but it's not really what the port is intended for.
I don't think it does. You could configure the HH5 to give the same dynamic IP address to the ASUS and then set the ASUS up as connecting to a static IP address on the WAN, and on the HH5 put the ASUS into it's DMZ and even turn off the HH5 firewall - that's about as close as I got.
Yeah, it's a shame ISPs aren't including proper bridge mode on their all-in-one VDSL modem/router offerings. At least there are some more reasonably-priced third party options now, like the Billion 8800NL and the TP-Link TD-W9980, for those not happy with the ISP-supplied kit.