How to collect information about a device or user connected to wi-fi?

Question

WiFi

There is an Asus RT-N12E router, I noticed that one device unknown to me is connected to it, changing the password did not help (10 characters large / small / symbols / numbers), there are literally a few unsuccessful connections in the logs and then a successful connection.
In the router settings, you can ban a poppy, but I would like to know who or what it is, how can I organize the collection of data about this connection?
There is no custom firmware for the router.

Answer 1

Check if you have WPS disabled. If it is enabled, then changing the password will not help you from hacking your network.

Answer 2

By poppy you can find out the vendor. Using a sniffer, see who and where goes with the IP that this device receives. If you're lucky, you'll catch the ID in social networks =)