At the firewall level, port forwarding forwards traffic bound for one port to another machine on your network on an arbitrary port, but the UI built on top of it in your router may not include this.
If it’s not an option in your Fritzbox, your options are:
Make the service running on your internal network listen on one of those high-number ports instead.
Introduce another machine on the network that also performs NAT between your router and your machine
Try to access the underlying firewall in your router to tweak the rules manually. Some routers have an admin console accessible via telnet or SSH that may allow this.
Get a new router.
The first and last options on this list are probably the best.