Tvheadend

Hi Guys, long time user, first time here on the forums.
I have a question regarding tvhclient that I just can't seem to crack myself!
I want to configure the client so it can be used over a 3G connection, or from anywhere outside my LAN. I have managed to get tvhclient working within the LAN when connected to wifi, but can't seem to get it to connect via 3G.

I've tried and tested all the usual things. My router is forwarding the ports out coorectly, and my DNS service from the router is correctly forwarding port 9981 onto the internet. I just cant seem to get tvhclient to connect to it? My access control has also been checked.
Am I missing anything obvious here?
Thanks
Adam