On your 3G connection are you connecting to the IP address of the system that works fine on your LAN connection?
If I read it correctly I think you have a NAT issue on your router....or you need to use the public IP of the router that is port forwarding instead of the internal LAN IP.
Can you post bogus IPs to explain your setup if I misunderstood? Also is the router you specified of the Cisco variety, if a different company/model that would be helpful too?