Hi @Paul_Steeghs,
This user has a similar router to yours - I’m hoping the GUI for the KPN 12 is similar to the V10 listed in your own description.
Provided you also share an internet service provider and have related account implementation in place, I recommend you attempt to create a manual port forwarding rule in the Access Control section of the router web GUI as they did here:
Please let me know if this helps. If not, please provide the details kindly requested by our helpful moderator above, and we will proceed from there. Thank you!