Cant get DynDNS to work properly....

Hello everyone.

Disclaimer, i basically just started getting into this selfhosting thing and really am at a limit right now.

I saw a very cool video from a german creator which demonstrates how 'easy' it is to setup dyndns for your own server since i dont have a public ipv4 from a provider.
(The Video for reference).

So I have a ubuntu server running in my network with Nginx Proxy Manager up and running through docker.

And as the video shows i tried setting up a domain via the service Ipv64.
In there a AAAA - Record was automatically created with the destination pointed to the ipv6 of my router.

In my Routers settings (fritzbox) i then turned on dyndns and entered the Domain Name, Credentials and Update URL.

I also turned on portforwarding for http and https inside of the router to my server.

After saving all the changes i went to Nginx Proxy Manager and just like in the video tried creating a ssl certificate. When entering my domain, and testing the Server Reachability following error pops up:

 There is a server found at this domain but it returned an unexpected status code Connection timed out.. Is it the NPM server? Please make sure your domain points to the IP where your NPM instance is running.

Then i thought, ok i will try to generate a ssl certificate while adding a proxy host, which still didnt seem to work but the proxy host was created. I then tried accessing the host i just created and this comes up:

The DNS rebind protection of your FRITZ!Box rejected your request for security reasons.

I then tried to access it from an outside network, because i googled for that problem and it seems to occur when trying to access that domain when in the same network. But well from other networks i cant access it at all, like the server cant even be found....

I really hope this explanation isnt to messy, i am so confused right now.

What could i be missing? What did i do wrong?

Help would be much appreciated <3