Smart home devices (light bulbs, plugs, sensors) are notoriously picky. Older U6-LR firmware would sometimes drop 2.4 GHz legacy clients (802.11b/g). Version 6.6.x introduced a "Legacy Device Compatibility Mode" that resolves disconnects for Tuya, Shelly, and older Amazon Echo devices.
If you want, I can:
Updating the firmware on a u6lr device is typically done through the UniFi Network Controller software: u6lr latest firmware