Port setup
See the Wikipedia on Port Forwarding to learn more about why you need it.
ssh
uses port 22 by default. I will assume you have not changed that. If you changed that port, then you will need to make changes in the instructions below accordingly.
Router setup
Make sure port 22 is forwarded to the Ubuntu server's internal IP address in the home router. This will allow anyone from outside the home LAN use ssh to connect to the home computers.
The instructions are router specific and beyond the scope of this answer as it has nothing to do with Ubuntu.
Some routers allow specifying both incoming port (from the Internet) as well as outgoing port (to the internal IP address of the LAN for the Ubuntu server). In that case, you may want to use a different port, say 2200, when you connect from the outside, and setup your router accordingly:

The picture above is just an example. The settings in your router may look different.
If you have changed the default port in /etc/ssh/sshd_config
then you will need to set the internal port above to that port, instead of port 22.
To learn how to setup ssh
to work within the home lAN, see Connect two computers with SSH in a home LAN
Here is another port forwarding: how to guide for reference.
Hope this helps