Recently went to setup a reverse SSH tunnel and had a few issues such as connections not being allowed, or otherwise. There is also a strong gotcha not documented clearly on the Internet, you can only have one connection per port on the intermediary host. If you want more than one host, you need to use multiple ports; sorry this is the way it is.
Before starting, make sure on BOTH hosts, install:
sudo apt-get install -y openssh-server