DIAGNOSIS Bluetooth connectivity issue between the remote and the stick. The infrared (IR) portion for TV power/volume remains functional as it's a separate, direct signal.
FIX Re-pair the remote. Step 1: Hold the Left directional button and the Menu button (three lines) simultaneously for 15 seconds. Release. Step 2: Wait 60 seconds for the remote to fully power down. Step 3: Remove the batteries from the remote for 30 seconds, then reinsert. Step 4: Hold the Home button for 15 seconds to initiate pairing. The remote should appear in 'Available Remotes' on your Fire TV screen. Select it to pair.
IF THAT DIDN'T WORK Interference from USB 3.0 devices or crowded 2.4 GHz spectrum. If you have a USB 3.0 hard drive attached to your TV or a nearby PC, move it farther away. Also, power cycle your router and any nearby Bluetooth speakers or dongles.
HEADS UP The remote's Bluetooth radio is weak. Ensure there is a clear line of sight between the remote and the Fire Stick. Obstructions like a coffee table or AV cabinet door can cause lag. The HDMI extender improves this.