The bluetooth module of the Dell XPS 15 9550 has never been working for me, but copying a firmware blob onto my machine fixed my issues.
Why doesn't this work automatically?
The firmware blob that is needed by Broadcom devices is not supplied by default, and it has to be supplied manually.
Download BCM-0a5c-6410.hcd and copy it into
/lib/firmware/brcm/ and then reboot your device.
wget https://memcpy.io/files/2017-10-28/BCM-0a5c-6410.hcd sudo cp BCM-0a5c-6410.hcd /lib/firmware/brcm/ sudo chmod 0644 /lib/firmware/brcm/BCM-0a5c-6410.hcd sudo reboot