At host behind the firewall :
autossh -M 20000 -f user@remotehost -R 4567:localhost:22 -nN
At remotehost, will listen to port 4567.
To access the forwarded port , at the remote host :
> ssh user@localhost -p 5678
Reference: https://forum.proxmox.com/threads/proxmox-ve-screen-out-of-range.131297/