Edit /etc/bluetooth/input.conf and remove the # commenting out the line so the line
If your input.conf does not have a copy of this comment for you to modify, try simply adding the line above.
Prior to doing this, my Logitech M557 Bluetooth mouse required that I manually connect it after every restart of my XPS 13 running Linux Mint 18.1 KDE. In addition, if the system was left idle for 10 minutes or so the mouse would cease working even though the system showed it to be connected. I had to restart the system to reconnect the mouse, since attempts to connect within the session were unsuccessful using either console commands or Bluetooth GUI.
Once I made this change to input.conf the mouse connects automatically across restarts and also stays connected during a session whenever I let the session go idle.
Best of Luck solving your issue.