First off, did you read How to use Synology NAS as sync store? It should describe how to set up a Synology NAS as sync store via WebDAV.
Secondly: your description a bit confusing.
- What is your NAS’ IP address? (Frankly, there’s no point in disguising a private IP address with something like “xxx” – that’s more confusing than helpful).
- What do you mean by "the database is located in a folder like …? This seems to be a networked folder, but it is discouraged to put databases on anything but a local disk (note: I’m not talking about sync here but about the location of the database!)
- What is the name of the folder you want to share on your NAS? A screenshot of your Diskstation’s layout might be helpful here.
- Did you set up a user who has read/write access to the folder on your NAS that you want to access via WebDAV?
- Did you change the default port of WebDAV over HTTPS?
- Can you connect from the Finder with “Go to server” and "https://:5006/ and your username/password for the NAS?
This is less than helpful. If there are relevant error messages, post them. If the error messages are not relevant, why mention them at all?
Which protocol, where? Mac? NAS? somehwere else entirely?