DIAGNOSIS The red LED flash is a general protection mode, almost always triggered by an electrical fault in the connection to the subwoofer. The soundbar shuts off audio to protect its amplifier.
FIX Power cycle the entire system to reset the wireless link. Unplug both the soundbar and subwoofer from power for 5 full minutes. Plug the subwoofer back in first, wait for its status light to stabilize, then plug in the soundbar. They should auto-pair.
IF THAT DIDN'T WORK The issue is likely physical proximity or interference. Move the subwoofer closer to the soundbar (within 10 feet, no major obstructions) and away from large metal objects or routers. Press and hold the ID SET button on the subwoofer's back for 5 seconds to force a re-pair.
HEADS UP If the red flashing persists after a successful re-pair, the subwoofer's internal amplifier may be faulty and require service. The soundbar can operate without the sub, but you'll need to manually disable it in the SmartThings app sound settings.