The easiest way is to run some kind of automated job (cron job), depending on the NAS you have, you can run some basic Linux commands, like rsync.
You need a way to connect the NAS and the Droplet, once you find that way, I think a plugin would be an overkill for example. You just need something like the code below, to run on your NAS, in a cron job
rsync -avz --progress -e "ssh -i ~/.ssh/id-rsa" example.com:/var/discourse/shared/standalone/backups /folder/in/the/nas
The ability to run something like this, would depend, as I said above, in which NAS you have.