Hello,
In order for port forwarding to work correctly, two more conditions have to be met (apart from just configuring Port Forwarding, which you have already done):
1. Your RUT955 must have Public IP address (You can check if your router has Public IP address, by comparing IP addresses, which router displays in "Status -> Network -> WAN" menu and any web page, which shows external IP address. Check second page of these short instructions for visual example how to check if router receives Public IP address)
2. Your Raspberry must have correct "default gateway" setting configured in its network configuration settings (If your Raspberry is using DHCP, than this is not an issue. However, if you have configured Static IP address on your Raspberry, make sure that "Default gateway" setting is configured as well. It must be set to router's LAN IP address (default 192.168.1.1))