Because of how/when Redux must run to work both embedded in themes and within plugins, a little massaging must occur if you want to set Redux values on plugin activation.
In short, if you’re not using the save_defaults argument you will have to manually set the values in the database.
So the typical set_option and get_option should be used on plugin activation matching your opt_name as the key.
If you let Redux do it all, then you need but set your default values and away you go.