If the links work for clients on the open internet, but don’t work for clients on a 3g connection, I would ahve to assume the problem is the 3g connection. There is no difference, from the server’s perspective, between clients in one network or another, they all come through the same port/interface. It is possible that the players are requesting the RTSP feed via UDP which is getting removed by the 3g network. You can try using links such as this on your mobile players to see if it helps:
rtsp://YourIP:5544/streamname?forcetsrtsp://YourIP:5544/streamname?forcetcp