Rather than doing that via private chat, I’ll post here what can be done – that might help others, too.
First, there already is a description on how to use a Synology NAS for sync here.
First of all, you need to register with a DynDNS service so that you have a (pseudo) host name which is mapped to the IP address of your router. If you’re using a Synology or a QNAP machine, you can go for their respective DynDNS services. For the following, let’s assume that you have a DynDNS address mynas.dyndns.com. Once you’ve set the DynDNS up, get yourself a certificate for this host from Let’s Encrypt and install it on the NAS. How that is done, depends on the brand, so you should look for details on the Synology or QNAP site or the site of whatever NAS you’re using.
For the following, I’m assuming that your NAS has the local IP address 192.168.1.128.
There are two possibilities to access the NAS from the outside:
- Port forwarding in your router. Easier to implement, but possibly less secure.
- Via VPN in/to your router. More difficult to implement, very secure.
Port forwarding
Assuming that you have your WebDAV server running via HTTPS on port 1234, go to the admin page of your router, find the port forwarding section and add a forwarding rule from the outside port 1234 to port 1234 on 192.168.44.128.
Change the DT WebDAV sync settings so that the URL of your server is https://mynas.dyndns.com instead of https://192.168.44.128. Do not change anything else!. Sync should now continue as before. If not, check DT’s log window on the Mac.
VPN
How you set up a VPN to your local network depends on your router and/or your NAS. So, check your router’s or NAS’s documentation for that. If it can be done in the router, that might be easier.
Use mynas.dyndns.com to connect to the VPN from your Mac and/or or i*OS devices(s). Then you should be able to use the original WebDAV sync settings, ie those that use 192.168.44.128 in the URL.