I have the newest version of Airport Extreme Base station and I have shared disks with it. I can connect to my disks via AFP with its IP when I am not connected to the local network while I can't do that with SMB. SMB share works fine on local network. Can someone explain why and whether it can be fixed? I want to use SMB because I use that to connect on my android. Unfortunately, there is no AFP client for Android.
I seem to come up with the same conclusion as you have. The AirPort Extreme can share out an AirPort Disk to the local network and be accessed by network clients using either the AFP or SMB file sharing protocol ... but when attempting to do so from a remote location only AFP is supported.