Trouble with WebDAV over internet


I have a Synology Diskstation to which I connect through Synologys own apps. However I am looking for an alternative for it and since I have Infuse on my Apple TV I wanted to try the app for iOS

I can connect through WebDAV from the app, but it often have cannot play my files despite being mpeg4 with AAC sound. Sometimes it will also refuse to connect to the share at all.
All port forwarding has been set through the Router and appropriate firewall rules has been implemented on the Diskstation. I have tried the app on both my iphone 6 and iPad Air and same problems on both.

What am I missing to make it work?