Have you configured the standard report of your Smart Plug with a downlink ?
See “Configure a standard report on the consumption of the remote SmartPlug” Smart Plug
As TTN doesn’t manage class C, downlink configuration frame will be sent as response to the next uplink frame from Smart Plug.
To send uplink frame from your Smart Plug, you can put Smart Plug in ReAssociation mode, either :
press 3 times the user button
or
unplug and re-plug the Smart Plug from power supply