You will need to set port-forwarding on the router that connects you to the internet.
for example (choose a port number yourself):
port 10022 on the router, forwarded to port 22 of your ubuntu ip.
(I would use a port > 10000 because some providers block standard ports, and 10022 reminds me it's ssh.)
Then find out your 'internet ip', with http://whatismyip.com
From somewhere else on the internet, then you can putty or ssh to that 'internet ip' on port 10022. What will be forwarded by your router to port 22 of your ubuntu ip.
So you will need to figure out how to configure that on your router. And have a good password, or not always leave that open ... for security reasons.
(Here port 22 is used to forward directly, what might not work or isn't advised to do so)