It depends on who has control of the systems and if you are allowed to install software on them. If so, one option is to use LogMeIn Hamachi.
LogMeIn Hamachi is a virtual networking service that can be set up in
minutes and enables secure remote access to your business network
anywhere there is an Internet connection.
Basically, you install the hamachi client on each system that you want to be connected. The client will allow you to create your own private VPN which will exist outside of your LAN. You then connect each of your systems to the VPN you just created. Each system will get its own hamachi IP address which you can then use to connect to it, like so:
$ ssh <hamachi_ip>
LogMeIn Hamachi is a paid service, however, it is free for up to 5 clients. There is a link to a PDF "Getting Started Guide" at the bottom of this page. I have been using hamachi for years and it works very well for my basic needs.