The routing device that has the public IP address needs to forward port 22 traffic to the Ubuntu server that is behind the firewall. If you tell us what kind of firewall / edge device it is, we might be able to help point you in a more specific direction.
EDIT: You'll need to check that traffic is truly being sent to your Ubuntu server. Run tcpdump to see if SSH traffic is hitting your server. If it's not, you still have an edge router problem. If it is, then you can start focusing on the Ubuntu server. Once you figure that out, we might be able to help you more.
EDIT 2: If you set up port forwarding for port 22, the router handles the translation for all traffic bound for port 22 directed at the public IP address and immediately sends the traffic to the Ubuntu server. The Ubuntu server then sees the traffic coming from the router, and any responses to port 22 traffic get sent to the router which then forwards it on to you. The router handles all the traffic translation by maintaining a table of who is communicating to which port and assigning unique sessions to each communicant. That way even multiple people can talk to the Ubuntu server through ssh using the external IP address.
For example: If you have an external IP address at home of 18.104.22.168 and your office router has an external IP address of 22.214.171.124 and the Ubuntu server has a local IP of 192.168.1.10 (behind the 126.96.36.199 router), the traffic flows like this:
- 188.8.131.52 connects to 184.108.40.206:22
- 220.127.116.11:22 gets forwarded to 192.168.1.10:22.
- 192.168.1.10:22 responds to the router at 18.104.22.168:22 (well, in reality it responds to the local IP address of the router, not the public IP address. It would probably be 192.168.1.1:22 in this example.)
- The router then forwards the information back to your home's public IP of 22.214.171.124.