Ensure ‘Enable backup rsync service’ is enabled in your Ugreen Control Panel, and select an existing user that will be used when transferring with rsync:

To rsync a local folder to your Ugreen NAS, use:
rsync -avz ./local-folder/ user-enabled-for-rsync@your-ugreen-ip:NAME_OF_SHARE
Note that instead of using the path of a folder on a volume, e.g. /volume1/folder, if folder is the name of the share, use this as the target path without any absolete path prefix.
This is hinted at in this post, and while it seems to differ from how you’d normally specify a remote destination, this is how rsync on a Ugreen appears to work.

I hope that this works. I have really been struggling with this!
Worked great for me, from an aging Netgear ReadyNAS box to my new ugreen NAS
Ok but how to do it automatically at every change in your local folder ?