I have a Magic Mouse 2 - the type that can be connected to my laptop with a Lightning cable, and a Retina MBP 15-inch Mid 2015 running Sierra (10.12.3).
My mouse does not connect via Bluetooth. Power cycling the mouse, the computer itself, the bluetooth, and the wifi does nothing. I've also tried un-pairing the mouse as well. I followed every step of every answer on this already.
However, when I plug in the mouse with my lightning cable, the mouse is immediately recognized as my mouse, and even has a 88% charge on it. The moment I unplug the mouse, it is disconnected and unrecognizable.