DIAGNOSIS The S800B's Wi-Fi module struggles with certain router bands or congestion. It's dropping its connection to your network and thus disappearing from SmartThings.
FIX Connect it to your 2.4GHz Wi-Fi network only. The 5GHz band is not supported. In your router settings, ensure the 2.4GHz network has a unique name (SSID) and try moving your router closer to the soundbar.
IF THAT DIDN'T WORK A conflicting IP address or DHCP lease could be the cause. In your router admin panel, assign a static IP address to the soundbar's MAC address (found in SmartThings app > Soundbar > Information).
HEADS UP Using Wi-Fi for Spotify Connect/AirPlay 2 is more reliable than for app control. For stable app control, consider using Bluetooth mode instead, though you'll lose some features.