Add A Router To A Rj45 Based Network?

I live in an apartment where my internet is included in the rent. In all the rooms there is an ethernet socket buillt into the wall. I'd like to know how to connect a router to this for wireless internet. I have tried one router, but it buggers up the ip address and brings down the network for the entire apartment complex. I've done this twice now and don't fancy waiting another week for the technician to call out to reset things. I had a friend who lived in a simalar complex, rj45 sockets on the wall etc. He has a thomson router, which has a little black box plugged into the ethernet socket on the back of the router. Into this little black box goes the ethernet cable from the wall socket and bam, he has wireless internet, even though he has no access to the main router or switch of the apartment block. It's not failed once either. Both the router and the little black box need mains power.I think it was originally for a WIMAX connection. It's small, maybe 2"X3" inch?