So basically, you have
- a folder with PDFs on your NAS which you can access from the WAN via your iPad/iPhone. How? I.e. which protocol do you use (SMB, AFP, NFS, …?)
- and one more database(s) on your NAS which you want to access via WebDAV. That seems to work as long as you’re inside your LAN, but not when you’re outside.
If that’s true, one would still need more DETAIL to be able to help you. AFAICT, Zerotier is DDNS that also does port forwarding (that’s what I guess, since their website is deliberately obscure on that). So your NAS should have an external name which you should be able to look up in your Zerotier console. Does it?
It should also have a port number there - does it?
This port number should be mapped to port 5006 and your NAS’s ip address in your router. Is it?
If you enter the external name of your NAS and the external port number in the sync settings of your iPad (something like: https://mylovelynas.zierotier.com:xxxx/, where xxxx is the port number) and your correct user name/password for the WebDAV service on the NAS - what do you get on the iPad? Any error messages?
If your router provides any error or warning messages, what do they say?
What do the log files of your NAS tell you: connection attempt from the WAN visible? Denied because of …? Successful, but …?