First, enable WebDAV in Control Panel of your NAS. Refer to your NAS manufacturer manuals how to that. During this process you should set a Port Number for HTTP and HTTPS protocols.
If you want to access your NAS via a secure HTTPS protocol (recommended), then you need to setup an authority signed SSL certificate on your NAS and also install it on each of your mobiles and computers. Again refer to Google search or your NAS manual for a how-to. You can get a signed certificate free of charge at https://letsencrypt.org.
Make sure that your enabled WebDAV for your specific user account in the NAS’s Control Panel. Later you will need User Name and Password of this account.
Next, you should deal with the IP address. Your NAS has a local IP address inside your office/home Wifi network, but you can access the NAS via this address only as long as your device is inside the same network. To access your NAS from outside (via a mobile network), your Wifi router should have a static IP address from your Internet Service Provider. And you should set the port forwarding on your router to/from NAS/Internet for the ports you set earlier for WebDAV access. Use your static IP as a Host parameter for the WebDAV sync.
A static IP address is required for a normal work of the WebDAV sync. There are a lot of dynamic DNS solutions including ones from NAS manufacturers. They won’t work.
Finally, you should figure out a Local Path to your user account’s home folder on the NAS. Examine the file tree on your NAS. This can be home or homes/user_name. And you can append some additional sub-folder where to place the database file, e.g. home/SafeInCloud.
Now you have all the required settings for the WebDAV sync. Try it out! If this don’t work, you are on your own - remember you should be a true power user. Support won’t help you cause it’s impossible to figure out remotely what’s wrong.