Iphone tethering with Xbox 360

Hello, I have an Xbox 360 Slim and an iPhone 4 with unlimited internet. When I used to share my 3G connection with my Xbox, everything worked very well. Now I have connection issues with my Livebox, and I wanted to go back to using my iPhone's 3G connection, but now it doesn't work. When I test my connection on my Xbox, it gets stuck between my iPhone and the internet even though my iPhone is connected to 3G and I have full bars, and I've tested it on Safari, everything works.

Thank you for reading.

Configuration: Windows 7 / Internet Explorer 9.0