The first question about the ADSL USB modem - what kind of device is it, is it more of a modem than a router? Have you searched that manufacturers website for port forwarding to ensure that its actually open?
Check
http://beta.hackerwhacker.com/freetools.php
Use trace port see if your port is open or not.
Cleverman wrote:
If anybody has some sure answers as to the Router-Router / Network-Network connection.
If you are asking how to connect Network A to Network B via VNC its just as you say for the ADSL Gateway/Router/Modem.
Firstly - Decide on a port - thats not blocked by your ISP. Configure the port on each server machine. You also have to set up the port (Default being 5900 viewer and 5800 web) to be passed to the proper computer through the router. e.g. Port 5900 TCP/UDP to 192.168.1.101 and port TCP/UDP 5800 192.168.1.102. -This allows two servers on a single network to be controlled by VNC viewer and the other by web viewer.
Secondly - check the os and firewall software on the "Server" Machine(s). Ensure the port that you are using is not being blocked. By windows XPSP2, which I hear has some other interesting issues with many programs. During Troubleshooting phase sometimes turning off software firewalls on the server machine works well to find out if it works.
Thirdly - you should test some connections either from a dmz, direct internet connection without router (viewer side only), or hotspot. Whatever is more convinient. Connect to the network server's EXTERNAL IP Address - go to
www.whatismyip.com to find out what your external address is (Sounds like both of you seem to know how to get that anyway)
This is where your troubleshooting comes into play. If you think you have the ports all set up to forward and dont know why its not connecting from an item on the internet there are many places to check. When you hit this wall it may help to post it on this forum to see if we can clearly find something wrong with the setup. Include router type model and internal addresses only. We dont need to know what your external ip is.
Lastly you can test from behind the network B or A (respectivly) router to see if the port forwarding is set up on the SERVER network properly.
In most cases if you will only have one side as a viewer and one side as the server. The VIEWER side doesnt require port forwarding, in most cases. This is due to most manufactuares allowing the port to be opened by the router / firewall because it was initiated on the internal network, not external.
But I have gotten it to work just fine - from work (Network A) to home (Network B) and use it once in a while when I forget stuff at home.
Hopefully this is what you wanted if not just wasted a few mins of my afternoon
. This makes a nonsense for me, because I can browse the web and fetch the mail throught the ADSL. 
