Assume we have an intermediate server with static ip or domain.

On the destination computer:

ssh -N -R 9876:localhost:22 uesr@jump.server

This will establish a connection between the destination computer and the intermediate server, and forward all traffic from port 9877 on the server to port 22 (the default port for ssh connection) on the destination computer.

Also we have to add GatewayPorts yes to the sshd configuration (at /etc/ssh/sshd_config) on the intermediate server and restart the ssh service.

sudo service ssh restart

Then we can connect to the destination computer with:

ssh remote@jump.server -p 9876

sshReach.me and SERVEO provide such service.

For SERVEO, use:

ssh -R myalias:22:localhost:22 serveo.net

and

ssh -J serveo.net user@myalias