Nextbox - problems with external IP address

Using the Nextbox with an dedyn Address for some time now. Worked fine for years.
But now we switched from Telecom-DSL to DNS-Net fiber. I use our Fritzbox to establish an dynamic DNS connection and that works so far.

But the Nextbox APP got a problem, wenn I activate the static domain configuration (since the Fritzbox takes care of the dynamic DNS) I got the notification:

Failed resolving: *** need: 79.140.###.### found: 100.110.###.###

I looked around a bit and it seems that DNS-Net uses shared IPv4 address. So the Fritzbox can deal with it and got as an external IP-Adress the 100.110.###.### address (and so dedyn points to that address) but if I use a webservice to ask what is my ip address I got the same as the Nextbox APP: 79.140.###.###.
I’m not mad about that my real address is hidden among others but it seems that the Nextbox is not reachable from outside the local net, as long as it expects the wrong external IP address. I looked up a way to get the right IP address and I made a scrip to ask the Fritzbox to return his external IP address to me. So, is there a way to convince the Nextbox APP to ask the Fritzbox for the external address (or use my script) and not using whatever webservice it uses to get the information about its presumptive external address? Or did someone knows another workaround for that problem I didn’t think of yet?

I know IPv6 will solve all of this problems but it seems that DNS-Net not providing native IPv6 support (yet) or I didn’t found out how…