This article provides the information on "Dell AD211 Bluetooth speaker will not automatically reconnect after Chromebook reboot".
AD211 Bluetooth Speaker Not Connected After Reboot
Chromebook users may notice their AD211 Bluetooth speaker will not auto reconnect after their system has rebooted.
Solution
This is a limitation of the AD211 speaker design and the Chrome operating system. After any reboot of the system, press and hold the Bluetooth button on the speaker for two seconds to manually sync the speakers.