the thinner the material the better, the more antennas it has the better the router on that aspect
if you need a shelp, wooden shelp, half incho wood or inch thick
if you can, no shelf at all, usually routers have in the back two holes, there you can hook two screws screwed to the wall, or even nails, i used nails because my wall allows nails, i could use screws too but nails was simpler and faster
with screws