How to Setup Reverse SSH Tunnel on Raspberry Pi behind a 3G USB stick modem

There are cases when a remote computer could be hard to reach. This computer could be placed behind a firewall or a router (NAT) whose rules or settings cannot be changed. An easy and practical solution that could help us in this case is to set up a reverse SSH tunnel on Linux. Reverse SSH tunneling allows you to create a connection from the remote computer to a local computer