You assume a lot but let's dismiss that. Not many routers can do this but the first link should help you begin your research to find routers that can do this.
http://www.google.com/search?&q=wifidog pops up fast enough and you can see if you can bear the work of flashing a supported router.
I know once in a while folk want to do this on their router or want a ready to use or "easy" solution. I can't tell which applies to you.
But I think I can help you begin your search.