I did find when I was doing it that it seemed to take a while for the fritzbox to show a green light next to the port forward rule and also took a while for my plex client to find the server. I would try set all ports (external and internal) to the default 32400 on both the plex server and fritzbox rules... then reboot both the fritzbox and plex server and then leave it a while and retry... Probably snake oil advice but that's what worked in the end for me, lol. Once it's working on the default port then maybe think about custom ports.