Cannot connect to a Mac via 3G

Cannot connect to a Mac via 3G

Device - iPad
Device OS version - 3.2.2
Computer OS version - OS X 10.6.5
App version - 2.0.7
Connection method - Manual-VNC
Environment of the remote PC - Home behind a Linksys WRT54G

I have searched through this forum and the online Help and User Guides but still cannot connect to my Mac from my iPad when connected via 3G. When connected to my wifi network it works great. I have reviewed the port forwarding instructions and even though this is a very old router it has a section for this and I have set it up for RDP port 3389 (start and end). I also created a separate connection in the app for the 3G connection that has the external IP address and I have tried using both connections via 3G and neither work. I also tried using both port 5900 (default) and 3389 on the "external" connection. What else can I check, what am I doing wrong? I am sure there is a solution just can't find it right now.