Nevermind. After writing my post I found this thread: No bluetooth in new Librem 15 v3
In it, it says that Puri.sm doesn’t load non-free code on it’s laptop, and since the bluetooth hardware in the Librem doesn’t have free drivers it doesn’t work out of the box. To install non-free drivers for it, I did the following:
1 Add the non free repo.
echo 'deb http://httpredir.debian.org/debian/ stretch main contrib non-free' >> /etc/apt/sources.list
2 and then install the package:
sudo apt-get update && sudo apt-get install firmware-atheros
After the driver has been installed, switch the wifi/bluetooth hardware switch off and on, and then it works.
I wish you all a beautiful day!