I have the same phone and a DIR-628 router and the same issue the KM900 works with my company's cisco router and 3 different Trednnet routers that I have substituted in place of the DIR-628....
I have a WAP for the basement (steel stud construction) and the phone connects to it just fine but still gets jammed at the router
Tried doing different firmware on the phone and router still no go.
This is definately a router issue and not a phone issue