Put this into your .bashrc to auto-start a tmux session after SSH login

auto-start-tmux-ssh.txt

Put these lines into your server's .bashrc:
##
## TMUX auto attach
##
if [ -n "$SSH_CLIENT" ] || [ -n "$SSH_TTY" ]; then      # if this is an SSH session
    if which tmux >/dev/null 2>&1; then                 # check if tmux is installed
            if [[ -z "$TMUX" ]] ;then                   # do not allow "tmux in tmux"
                    ID="$( tmux ls | grep -vm1 attached | cut -d: -f1 )"    # get the id of a deattached session
                    if [[ -z "$ID" ]] ;then                                 # if not available create a new one
                            tmux new-session
                    else
                            tmux attach-session -t "$ID"                    # if available, attach to it
                    fi
            fi
    fi
fi