DIAGNOSIS This is often caused by band steering (Smart Connect) getting confused with older devices, or the router's "Airtime Fairness" feature which can unintentionally punish slower smart devices.
FIX Log into the router's web interface. Go to Wi-Fi Settings and turn OFF "Smart Connect." This stops the router from trying to move devices between bands, which many smart plugs and sensors can't handle. Create separate, dedicated networks for your 2.4GHz smart devices. Also, under Connectivity > Advanced Router Settings, find and disable "Airtime Fairness." Save and reboot.
IF THAT DIDN'T WORK Your 2.4GHz channel might be super congested. In Wi-Fi Settings for the 2.4GHz band, change the Channel Width from "Auto" to "20 MHz only" and set the Channel to 1, 6, or 11—whichever looks least crowded on a Wi-Fi analyzer app. This gives a slower but far more stable connection for IoT gear.
HEADS UP Some smart devices have truly terrible Wi-Fi chips. If one specific device keeps dropping, assigning it a static IP address (in Connectivity > DHCP Reservation) can sometimes help it stay put.