Diagnosis
HDMI-CEC handshake failure. This is a common issue with all HDMI ARC/eARC devices after a TV update or power cycle.
How to Fix It
Power cycle the entire chain. Unplug the soundbar and your TV from the wall for 2 full minutes. Replug the TV, then the soundbar. Turn them on in the same order. Use the HDMI cable that came with the soundbar.
If That Didn't Work
Disable and re-enable CEC on your TV. The setting name varies (Samsung: Anynet+, LG: Simplink, Sony: Bravia Sync). Turn it off, confirm the soundbar loses ARC, then turn it back on.
Heads Up
The Bar 300 must be connected to the specific HDMI port on your TV labeled 'HDMI ARC' or 'eARC'. A regular HDMI port will not work for audio control.