I think things are going to be a bit more complex than you think.
First off your Computer Router has no idea where you PLC Router is, and unless you have a fixed ADSL IP address it will change as and when your service provider feels like it. So you will probably need to register with a Dynamic DNS provider, there is hopefully a settings in your PLC Router that will allow this.
Next problem can be that some ADSL providers block all ports except those that are normally needed, they do it to for your security. You may have to unblock the ports you are using by visiting your ADSL providers web site, logging in and changing settings.
Then to test the link I would set up port forwarding on the PLC Router on a port to forward to the PLC on port 80. Set up a simple web page on the PLC and from your Computer, by typing in the Internet IP address or Dynamic DNS address (plus forwarding Port) of the PLC Router you should be able to connect to the PLC web page.
That is great for web pages but your PLC programming software needs to know how to connect to the PLC, I think you would have to set a Static route in your Computer Router or on your computer so that it knows how to connect to your PLC Router. Then you should be able to connect.
You can see why most of us go for a simple device like this:
http://www.e*w*o*n.biz/en/e*w*o*n-cosy-141.html?ewp=33
(take out the *, the server here blocks the name of the device).
You start the software connect to the new Router and your PLC gets an address on the same LAN and the PLC. Your PLC software won't know and won't care that it is a remote connection. It is more expensive, but way easier. Also Port Forwarding opens holes in your firewall that other could find and exploit.
