It would probably be cheaper than that zyxel thing to set up a wireless netwrok through a cheap firewall/router/server machine.

That way you can log who accessed which sites and when. You'd have remote access to the machine for adding and removing allowed devices and sites.

I'm sure you will be able to find a free list of dodgy sites to block.

You could prob make a cheap linux setup for a couple hundred quid. Don't need a monitor, keyboard or mouse to have it running.

If you don't mind a little more work

Free clicky