What works: LAN/SMB connection to NAS from within home network

What doesn't work: LAN/SMB connection to NAS from outside home network via "myfritz" DDNS by the router and forwarding port 5001 to the NAS. The address that works (!) in a browser is something like this https://somecrypticcharacters.myfritz.net:5001 It also works in the NAS app "DS file" of Synology. But Solid Explorer can't find the host. I've tried different combinations of the ports 5001 & 445, both in the address line and in the extra port field. Nothing worked.

What am I doing wrong?