Hello Tomtom

                        We understand that you are trying to do port forwarding on the modem interface, since you have done with all steps mentioned on the modem settings we would suggest you to restart the modem for a minute then try the steps mention on the link below and also make sure that the port number which you need to open are available or not using the port checker it make sure that the port is available to open.

                          http://arris.force.com/consumers/articles/General_FAQs/SBG10-Port-Forwarding-Setup

Let us know if this helps.