Diagnosis
Many smart home devices have weak Wi-Fi antennas and are sensitive to interference. The BE800's powerful radio might also be causing issues if it's set to a wide channel width they can't handle.
How to Fix It
Log into the router's web interface at tplinkwifi.net. Go to Advanced > Wireless > 2.4 GHz Wireless. 1) Set the 'Channel' to 1, 6, or 11 (these are non-overlapping). 2) Set the 'Channel Width' to 20 MHz only (not 'Auto' or 40 MHz). 3) Ensure the 'Mode' is set to something like '802.11b/g/n mixed' to ensure maximum compatibility. Save the settings and reboot your smart devices.
If That Didn't Work
The router might be too far away or blocked by walls for those low-power devices. Consider moving the router or the smart device, or adding a simple Wi-Fi extender dedicated to the 2.4 GHz band for that area.
Heads Up
Avoid using the 'Smart Connect' feature if you have lots of IoT devices. Keeping the 2.4 GHz network as a separate, stable, and slower network is actually better for their reliability.