If we have:
A) public server with static IP address (example some rented Linux VPS with IPv4 12.123.1.250)
or
B) server with private IP in some LAN (home computer example)

Both have Linux, or virtualized Linux via Virtualbox for example, then we can achieve private server be accessible from internet via our public server IP thanks to SSH and its reverse tunneling.

First make sure SSH is enabled, running and starting at boot:

redhat Linux (example CentOS):
chkconfig sshd on
service sshd start

debian Linux (example Ubuntu)
systemctl enable ssh
systemctl start ssh

>>> Execute on private server: <<<
ssh-keygen
ssh-copy-id [email protected] -p PublicServerSSHPortNumberUsually22
(sending public server key for password-less access to this local, private server)

Debian based Linux: apt-get install autossh
Redhat based Linux: Google your Linux distro and install autossh to install autossh

Establish reverse tunnel to the public server:
autossh -fN -R 7000:localhost:22 [email protected] -p PublicServerSSHPortNumberUsually22

If no error, then make that command run at boot:
echo "@reboot root autossh -fN -R 7000:localhost:22 [email protected] -p PublicServerSSHPortNumberUsually22" > /etc/cron.d/reversetunnelstart && chmod +x /etc/cron.d/reversetunnelstart

>>> Execute on public server: <<<
ssh-keygen
ssh-copy-id "127.0.0.1 -p 7000"

To login private server (example home computer) from anywhere in the world:

1) login your public server: ssh PublicServerIPHere -p PublicServerSSHPortNumberUsually22
2) once loged into public server (which has established connection to private), you can login private by cmd: ssh localhost -p 7000