I have a Synology NAS that was setup to sync with Dropbox but stopped syncing back in 2018 and I haven't noticed until now. Since then, I have been working on my files exclusively through Dropbox but would like to reconnect my NAS server back to my Dropbox account. It's obviously been a couple of years, so the files and organization that exist on Dropbox are the most up to date versions. How do I successfully sync what is on my Dropbox to my Synology NAS without causing the old files and folders from my NAS to sync to Dropbox and instead have the files and folders that are on Dropbox be what shows up on my NAS?
Since the Dropbox folder on your NAS server has been disconnected for a long time, the best method I could suggest to ensure that only your new data sync down from your account would be to resync the Dropbox folder completely.
To do this, you can unlink the device from your account security page here.
Before reinstalling/relinking the desktop app you'll need to rename the old Dropbox folder on your NAS (eg. Dropbox (Old)).
When you reconnect your account to the desktop app you can choose to sync Dropbox again to your NAS and set it up as new. The app will create a new Dropbox folder and sync the contents down from your account online.
I hope this helps, let me know if you have any questions!