Airport Express can only extend network, not create new one. Why?

I am sharing a flat with some friends. The shared broadband router is placed in the living room. To strengthen the signal and make it easier for me to manage, I bought an Airport Express, connected it to the shared router with an ethernet cable and set up a wireless network of my own in my room. It was working fine, until recently we renewed our broadband contract. The router had been replaced by a new one (I don't know why) and the network configration had been changed. Since then I was not able to use my own network. In the Airport Utility display, the light of Airport Express is green, but the light of Internet is yellow, and the status is disconnected. I tried to change various other configuration on the Airport Express, and all in vain. Until I switched the network mode from 'Create a new wireless network' to 'Extend a wireless network', the Internet connection was finally back online. But this merely extends the existing network and not what I want. Why this happens? Why my Airport Express can only extend a network, not create one?