I have a small Minecraft server running on my Pi and I also have the ports on the FRITZ! Box released.
The only problem is that the IP changes all the time (IP on the Internet).
Can you turn that off somehow?
Dyn DNS, should be somewhere in your Fritzbox, or you can also work with myfritz and domain.
The public IP should not change "all the time", but between once a day and every few weeks (with an IP-based telephone connection).
You can get a static IP from the provider, but it is rather expensive.
Typically you just take a dyndns service and then go over the name.
Get a DynDNS and enter it in your FritzBox.
https://www.noip.com
Then you no longer call up the IP of the server, but the DynDNS, which then forwards to your current IP.
For this you need a DynDNS, I recommend duckdns.
https://www.duckdns.org/