Responsive select plugin: doesn't work for primary nav

I am wanting to use the plugin “Responsive select” which gives me a select drop-down when below certain widths, for use on a mobile.

I installed the plugin and activated it and it worked fine.

However, I have two menus on my site (Primary Nav which is marked as the primary navigation, and a supplementary menu).

Responsive Select has a settings page which is defaulted to “Apply the responsive select menu to all menus”

I unticked this. Instead I ticked the singular navigation I wanted to use:

Selectively Activate Theme Locations

Disable the above and activate only the theme locations you want. These theme locations correspond to the Theme Locations Meta Box in Appearance > Menus

Primary Navigation
ON

This tallies up with the navigation I have chosen as my primary within the menus settings page.

However when I do this it doesn’t work at all? I just get the old menus. But when I say “all menus” it does work for “all menus”.