morrisk. I have a similar setup with my modem (Technicolor) bridged to an AirPort Time Capsule (router) and also want to access stats and the modem interface even though DHCP is turned off off on the modem. To do so you'll just need to create a connection with a manually assigned IP address In Mac OS X System Preferences > Network you'll should add a new service (Ethernet or Wiresless) using the + buttom bottom left and assign a manual IP address for your Mac in the range 192.168.1.x. See screen capture below where I've chosen 192.168.1.50. Each time you want to access the modem it's then just a simple case of temporarily making your regular (DHCP) connection 'inactive' and make this 'direct' connection to the modem temporarily 'active'
