[Solved] Android App not connecting outside

[DominikFickenwirth]

Thank you for your quick reply!

I did not configure DynDNS on my Synology NAS, but on my router. When I check with this tool:
   

I can see that the A-Record (IPv4) and AAAA-Record (IPv6) both point to my router. I can also see that IPv6 connection fails.

Somehow I assumed that the IPv4 port forwarding from my router to my Synology NAS would also work for connections coming in via the public IPv6 address of my router. This is not the case: I don't have IPv6 configured on my network, so my Synology NAS does not have an IPv6 address. Therefore, the router would have to forward the incoming IPv6 request to the IPv4 address of the NAS (which it doesn't do).

As you suspected, my router does indeed block all incoming IPv6 requests. Also, in the settings of my router, it's not possible to force DynDNS to send only the IPv4 address.

I have now configured DynDNS directly in my Synology NAS instead of the router, sending only an IPv4 address. The Android app works without any problems!