By default the photoframe app searches servers in the local network which is fine for most of the users. But sometimes the server is not found or the server, you want to access, is in a different network (could be over VPN for example). In that case, you can enter the server name or its ip address in the options menu and then only that server will be shown. No network scanning is done at that point so the only server you will see is the one you entered.
Unfortunately this only works for the network option, not for upnp. If anyone knows how to get this working with the cling framework or just how it works with UPNP. I already found this interesting page explaining UPNP. Since it works with UDP unicast it doesn't seem to be easy to do. I should be able to skip the discover state and add a device with the fixed ip directly.