On the computer target behind a firewall, in which you want to login in the future, do

ssh -f -N -R user@server

server need to be reachable and needs to have the following option set in /etc/ssh/sshd_config

GatewayPorts yes

Now any client can reach target with

ssh -p 19999 user@server

This can also used for, e.g., sshfs

sshfs -p 19999 user@server:folder ~/sshfs