Diagnosis
It's not broken. The BE800's default settings often enable the new 6 GHz band (Wi-Fi 7) and a 'Smart Connect' feature that can confuse older devices that only support 2.4 or 5 GHz.
How to Fix It
You need to split the network bands. Open the Tether app on your phone. Go to the router's settings, find the 'Wi-Fi' section, and look for 'Smart Connect' or a 'Band Steering' option—turn it OFF. This should reveal separate networks named something like 'YourNetwork_2.4G', 'YourNetwork_5G', and 'YourNetwork_6G'. Connect your older devices to the _2.4G or _5G network.
If That Didn't Work
If you still can't see the separate networks, log into the web interface at tplinkwifi.net. Under Advanced > Wireless, you can manually configure and enable the 2.4 GHz and 5 GHz SSIDs there, ensuring they are set to 'Broadcast'.
Heads Up
Your Wi-Fi 7 devices will get the best performance on the 6 GHz band, but for compatibility, keeping the 2.4 GHz band on for smart home gadgets is a good idea.