An alternative...

[JMilesT]

If you use a service like DynDNS, you can leave the mobile device set to the URL provided by DynDNS all the time.

When you're in range of your LAN, you will get Wi-Fi speeds, and when you're away you'll get 3G/4G speeds. You won't have to switch between a local IP and a Public IP in your Subsonic client.