Update - yes, just tried. I can access the SSH on Synology from an external computer with my home network’s DynDNS web address. It is just that when connected to the same LAN as Synology, Duplicati does not accept the web address, but wants the ip address instead. Both local and external ip-addresses do work.
(Yes, I think so. TCP is forwarded to the port I chose on Synology for the SSH. Since the external ip-address works, doesn’t that also suggest port forwarding is ok?)