@DDD and @meme
I'm not a professional, but I assume your Diskstation must be available via internet.
I recommend to set up a free account at https://account.synology.com/.
Once connected you need to ensure the required ports are forwarded by your router to your Diskstation, for WebDAV the port is 5006.
Furthermore you got to ensure to grant the specific user the rights to use WebDAV within the system controll settings of Diskstation (see screenshots, unfortunately in german).