Hey,
I don't think that you will get happy with that setup.
If you check the ddwrt wiki =>
http://www.dd-wrt.com/wiki/index.php/Asus_WL-520GU they state that the USB support is not that good.
I am also wondering why you had problems with the network settings. Last time flashing a router with *WRT, i think it was ddwrt or maybe tomato, this all worked out of the box including a webUI. Have you considered trying ddwrt?
Maybe you could post a link to the guide you followed and tell which step did not work for you.