To understand the diagram a little more. The blue line that connects the RUT955 to the modem, is it a LAN connection, or what kind of interface? I can't see well in the image the LAN IP of the RUT955, but I can see that it is 192.168.20.104, maybe? If it is that, and you have a LAN connection to the modem. You will not be able to access the modem because its IP is on another network. You must configure the LAN IP of the RUT955 as 192.168.100.2 for example, to be able to access the Modem from the RUT955 or any computer connected to the RUT955.