Ah, thanks! That did it 
Once I fixed up hostnames, and re-started Duplicati from within DSM (using hostname), the azure connection test worked. So not sure why it did not work first time…
Really appreciate the suggestions. I prefer to use within DSM, as the sole purpose of the box is backing up (it runs ActiveBackup for PCs, servers, ms365 and is a backup vault for photos). So I like to keep it all together.
So I recreated my “Photos to Azure” backup job - and pointed it to the same container where I had run my initial job from my Windows install. Duplicati detected the files on the destination and stated it needed to repair the local database. So I’ve kicked that off (about 10,000 “files” need to be updated into the db). If that works and means I can then continue runningthe incremental backups from DSM then Duplicati just went up another notch for me.