I think you have to use a Switch Item instead of a String Item. Then tag the Item with [ “Switchable” ] (I think that’s the one for Alexa). Then Alexa will send ON and OFF to that Item. You may have to use a Design Pattern: Proxy Item to convert the ON to “On” and OFF to “Off” that the String Item seems to require.
If this device is working with the Broadlink Binding, why do you need the Exec Binding Thing?