To automatically start tmux on your remote server when ordinarily logging in via SSH (and only SSH), edit the ~./bashrc of your user or root (or both) on the remote server accordingly:

if [[ -z "\(TMUX" ]] && [ "\)SSH_CONNECTION" != "" ]; then tmux attach-session -t ssh_tmux || tmux new-session -s ssh_tmux fi

This command creates a tmux session called ssh_tmux if none exists, or reattaches to a already existing session with that name. In case your connection dropped or when you forgot a session weeks ago, every SSH login automatically brings you back to the tmux-ssh session you left behind. Connect from your client:

Nothing special, just ssh user@hostname.