Check whether your 2.4 network is set to require Protected Management Frames or to use WPA3 mixed mode. Many older plugs cannot handle either and will fail to rejoin after even a short blip. Set security to WPA2 AES only and PMF optional. Also make sure band steering is off so the router does not keep nudging them between bands. Lock the channel and width and extend the DHCP lease. Simple changes, big difference.
