I have two ISP lines, one giving me a static IP and which I use for my VoIP phones due to better quality and lower latency with the drawback that I only have 8Mbit and the other one for better speed but higher latency and not so reliable.
As it seems today seems to be a bad day as the line was slow as a snail stuck on chewing gum and I really needed to do something on that server.
SSH Tunnels to the rescue
Now, to overcome this issue and connect from my fast line to the firewalled server I used some SSH tunnel magic :)
From the firewalled server I connect to a second server, reachable from the outside and create a reverse tunnel:
ssh email@example.com -R 2424:localhost:22
This connects you to example.com, opening port 2424 on example.com which is a tunnel back to the firewalled server (hence the localhost:22 part)
The issue is that the tunnel is only listening on localhost on example.com so this means that if you need a SSH Key you can't ssh from your local machine to example.com port 2424 ... so we need one more tunnel :)
Even more tunnels
Now from your local machine create a new tunnel
ssh firstname.lastname@example.org -L 2426:localhost:2424
This connects you to example.com opening a port 2426 on your local machine which is a tunnel to example.com port 2424.
Now on your local machine open up a new terminal, ssh to user@localhost -p 2426 and you will go through both tunnels and pop out on the firewalled server through the fast ISP Line and through SSH secured tunnels.