Browser is unable to connect to internet

Hello,
I live in an apartment building and I connect to the internet through a AT&T 3G connection card. I'm able to get a signal and have full bars for connection. When I open Firefox and try and load any webpage in my apartment I either get an error message saying Firefox can't find the server or the network connection timed out. However, the second I step outside my door into the hallway, which is not open to the outside it is enclosed, Firefox can load any page that I go to. Does any one have a clue why a webpage can't load inside my apartment but will load if I take the computer on the other side of the door?

Thanks