@ƁìĝŤ: To begin with, on your S21 Ultra, please press the Volume Down and Power buttons together for around 10 seconds until the phone restarts. Don't worry, this won't affect your personal data or settings. Now, please place both Buds in the charging case, close the lid for 10-15 seconds, then open it to enable Bluetooth pairing mode, and check your phone's screen for the connection prompt. During this step, please ensure that your Buds are within 10 metres of your phone, and there are no obstacles in the way that may cause interference such as thick walls or electrical equipment.
Furthermore, please ensure that your Buds are up to date by heading to the Galaxy Wearable app > About earbuds > Update earbud software > Download and install. You can also try heading to Settings > Connections > Bluetooth, and toggling this feature off, then on again to see if this helps. If the issue persists, tap the cog to the right of your Buds in the Bluetooth settings, select 'Unpair' then place your Buds in their case, close the lid, then open it to automatically start the pairing process. You can also try heading to the Wearable app > About earbuds > Reset earbuds. Let me know how you get on.
