You need to get your External ID from the phone and enter it in "Dial Number" on the N800's internet connection. Here are the instructions from the wiki.

"The Dial Number must be '*99***1#' - note that the '1' corresponds to the data account on your phone to use. Make sure it's right by going into (on your phone) menu->settings->Connectivity tab->Data communication->Data accounts, then select the right data account, then hit more->account info. On the second line is the 'External ID' parameter - this is the magic number that needs to go into the 800 'Dial Number' string."

Hope this helps.