You need a static public IP address or dyndns address to allow access to your home network.
Then in your home modem/router you need to forward the ports you'll be accessing externally to the ports on the Duo2 (default ports internally on the Duo2 are 80 for openwebif and API control, 8001 for full bandwidth streaming, 8002 for lower bandwidth transcoded streaming). In my case, I chose to use external port 85 forwarded to Duo2_LAN_IP:80 and external ports 8001,8002 straight through forwarded to Duo2_LAN_IP:8001,8002.
Then you can access the Duo2 remotely at your public_address:external_forwarded_port. You can use a PC with the OpenWebIF to control which in turn uses VLC to display. Over a 3g network you would require to set the transcoded bitrate down to 350Kbps typically (full bandwidth streaming certainly won't work) also reduce the resolution to say 320x240. These settings are supported through the OpenWebIF. When you change these settings, currently you need to do an Enigma restart to get them to be implemented... then you should be good. If you have a better remote network then you'll be able to get higher resolution/higher bitrates. In my case, I have an ADSL connection at home which is 8Mbps down / 384Kbps up. My Duo2 has to use the up rate to remotely serve... so I am quite limited.
There is also an android App called Vu+Player that is currrently being further developed which looks promising.
Good luck