Hi,
I had this type of issue a few times ago for my AI based security server. It is quite simple and does not require
a key.
On Home Assistant you could try to install sshpass and create a dedicated user on your external server:
sshpass -p [mypassword] ssh -N -R 8887:localhost:8123 sshtunnel@[my external server]
Note: I could install sshpass using the terminal :
apk install sshpass
on your external server I use a tool called “socat” where I redirect the ssh flow from the internal
interface througth the external one (can be launch at start, ie. in /etc/rc.local ) :
sudo socat -d tcp-listen:8888,reuseaddr,fork tcp:127.0.0.1:8887
-Franck