How is your Roon Core computer connected to your network? WiFi or Ethernet cable?
Can you go into your 3890 web administrator interface and confirm you see the Roon Core computer, the Apple TV, and the Ikea gateway? Are there any other devices connected? No WiFi extenders, nothing else?
It may be time to contact TDC and talk to them about what you are trying to do.
You mentioned that the Apple TV was connected via Ethernet cable, but that is not showing. Did you disconnect it before you took the screenshot? What device is the Ikea gateway?
Also, I see you have DynDNS as a configurable option. Is this active? I am not very familiar with it, but as it allows remote access, if it is active, then it may be part of the behavior you are experiencing.
Yes. After the 185.107.xxx.xxx address is an addtional 100.68.xxx.xxx address. It is this second address that demonstrated TDC probably is using GGNAT. When CGNAT is implemented by a service provider, there really is no way for home-based routers to work around it.
TDC may be able to deactivate it for you, or offer a static IP address in place of your CGNAT’d address combination. You will need to contact them and discuss options with TDC.