I never register to forums, but actually took the time to register to this forum because I was having issues getting DS Audio app to work. I have now solved the problem and want to share my knowledge. Mostly because the DS Audio app had little help in the app (like an example of what you should type in) and the answers on the forum seemed insufficient. So hope this helps.
1. Forward the ports on your router for your diskstation. Synology has great documentation on accomplishing this.
2. Register with DDNS service. Paid $6 for a year with NO-IP with a promo coupon. With free account you have to log-in at random intervals.
*Your external ip is what is registered with DDNS service, which in all reality will point to your router. If ports are not forwarded then your router config page will show up when you browse to your ddns.host.domain.
If your typing in 192.168.X.X and your on 3G then your just pissing in the wind because your pointing to your internal address. On your HOME wifi the internal address will work for the DS Audio app.
3. THIS IS THE ONE THAT KILLED ME FOR A FEW DAYS... "Network not available
... bla bla bla". What the hell does that mean?
I realized thru trial and error that http://
was not needed in front the the network location.
So for your ddns redirect type example: joesregistedddnshost.no-ip.com:5000
On your local network you would type example: 192.168.1.100:5000
4.Make sure the user you are logging in with has permissions to use the audio program on the synology server.
5.I've read that if your setup for https that it wont work... never tried it so dont know.