Ubuntu :: Wireless Router Configuration Page Not Showing All Devices On LAN

I went to see if the iPhone was connected and it wasn't listed--and she was now on my wireless network, on the Internet. For the past month, I have experienced slow Internet (the Qwest people came over three times to fix the problem, to no avail.) And, when my laptop and desktop are turned off, the modem still shows activity, as if someone was downloading large files. Qwest insists that my network is secure and no one is using my network (and my house was built in 1997, so the wiring is fairly new and I have had NO issues with Internet/phone and had Qwest service longer than I can remember).

To make a long story short, I am concerned that my router has been hacked. I live in a rural area, so there's no teenagers or weird-o's around that I know of that would do that (unlike city/suburban areas)--and my Linksys router is "lying" to me, as my nephew was ON MY NETWORK and the SSID, MAC address and IP number was not listed on the LAN device list--only my laptop/desktop are listed, and she was happily checking emails on my network. I have changed my WPA2 password three times just in case, no fix.