Sounds to me like the router is 'supposed' to be the gateway for your building / complex...  More or less, if you aren't paying for individual internet service, then that 192.168.0.x address is likely the NAT'd internal address, assigned via DHCP, from the gateway.  Unless they put a machine (one, as it can't do multiple) in the DMZ on the router, nobody will have a 'public' IP address.

Edit:  if anyone else sees that rogue router, then it sounds like the building isn't wired right, as each should have their own dedicated connection to the ISP.

Sounds like it's time to fire up a packet capture and see what's really going on

While I like that idea, that's a very dark shade of gray where the law might come in. Now, nothing stopping you from seeing what is COMING to your system. But Bob accidentally setting your interface to promiscuous may get you in legal trouble.

OK here's the deal. Residents plugging in a LAN port on their wireless router into the internet wall jack (supplied by the complex) used to be the bane of my existence in my former job. I used to hunt them down daily.

 You first need to know what the correct address scheme is for the internet provided by the complex. Frequently they will use something different than the 192.168.x.x network most home wireless routers use. If your wireless router is working, it is probably due to it still holding on to a correct address. Log in to your router and look at the WAN address from your ISP.

You can usually statically assign yourself an address (after ping sweeping the space to make sure you don't use an address already in use) if you keep receiving an incorrect address from the rogue router.

If you really want to, you could check out my video on dhcdrop on securitytube.net. It will use up all the addresses from the rogue router, making it more likely for you and others to receive a correct DHCP address.

 OR you could just inform your ISP and if they are worth a damn, they'll come out, track down the offender and disconnect them. Also, if they have left the password as default (as these kind of people usually do), you could log in to the D-link and turn off DHCP. I used to walk people through doing this over the phone to prevent a truck roll. Legal? It was for us because we were the ISP and they were breaking their TOS.

Yea, I get the point you're making. It's not that it's actually illegal; it's that people don't understand and will freak out. You just want to avoid a hassle. It's the same reason I'd never go tell a neighbor that their wifi is insecure and that I could theoretically see everything they're doing. I totally agree.