If you select a host in the network overview, there is an option Always assign this network device the same IPv4 address. If you tick that the address never cha

If you select a host in the network overview, there is an option Always assign this network device the same IPv4 address. If you tick that the address never changes. Also in modern Fritz!Boxes port forwarding is associated with a particular host, so I think it also works without the static assignment enabled? Anyway, I have logged on to my headless GPU machines remotely through port forwarding for years and never had an issue.