If you haven’t checked this yet, what Bluetooth version is running on your laptop? The user manual says that Bluetooth 4.0 or higher is required.
Also, here is the link to some debugging things you can try. https://fedoraproject.org/wiki/How_to_debug_Bluetooth_problems
One more thing, as I mentioned before, try making sure the Bluetooth is turned off on your phone and any other device that is near by. Only the Bluetooth on your computer should be turned on. Then try connecting your headphones. Do simple troubleshooting methods like: (1) Turn off the headphones (should be about 4 seconds of pressing the button), (2) turn on the headphones (should be about 2 seconds of pressing the button), (3) make sure both ear-buds are turned on and paired to each other, (4) turn off computers bluetooh, (5) turn on computers bluetooth, (6) with both the computer bluetooth and both ear-buds on, try pairing it with your computer. Again, make sure that there are no other devices that have bluetooh turned on. If you ever paired your ear-buds to your phones, it will not pair to the computer. You must turn your phones bluetooth off or delete the saved pairing in your phone and then turn your phone bluetooth off.
If it still does not work after messing around with some of those troubleshooting methods, try resetting your ear buds. While the earbuds are charging (solid white lights), hold both power buttons for 5 seconds or until the blue/white lights flash 3 times and turn off. Then take the buds out and put them back in the charging case. Make sure you delete any saved pairing from your phone or other devices. Then to activate them, take out the left earbud first, then the other. Then try connecting your earbuds to the computer.
But, this is all assuming that the Dell XPS 13 has bluetooth 4.0 or higher.