They don't actually say why. All they say is that "the following routers have been found to be incompatible with XBL".

I've done all the workarounds with telnet etc but still I get cut off after 5 minutes of play.

If I connect directly to the modem theres no issues.

See here: http://www.xbox.com/en-US/support/connecttolive/xbox360/homenetworking/equipment.htm