Open Terminal and type bluetoothctl
There one can input help to get a list of available commands.
Turn the power to the controller on by entering power on It is off by default.
Enter devices to get the MAC Address of the device with which to pair.
Enter device discovery mode with scan on command if device is not yet on the list.
Turn the agent on with agent on
After this things are changed: You will have to experiment a bit.
My guess is the bluetoothctl just goes on on its own here.
pair MAC Address to do the pairing (tab completion works). If using a device without a PIN, one may need to manually trust the device before it can reconnect successfully. Enter ‘trust MAC Address’ to do so.
use connect MAC_address to establish a connection.
Your mouse needs full batteries and power on mouse must be on. Play with the mouse to keep it busy. Connecting could take some time.