Can’t replace my left nav sidebar with this widget

I’m trying to use Advanced Sidebar Menu to replace the built-in left sidebar nav menu that came with my theme (it’s a custom theme…still trying to reach the developer). The native one doesn’t work properly and after days trying to fix it, I’m now just trying to get rid of it and use this plugin.

When I add the Advanced Sidebar Menu widget (and have tried many others) to the left sidebar area of my widget page, always, the native sidebar overrides my change.

I wonder if it’s because the theme is calling for the native sidebar before it can call the widget. But I’m not well-versed enough in WordPress to know if that’s true, or how to fix it.

The page I need help with: [log in to see the link]