Keep checkout on WordPress

I am having an issue with a new site using this plugin. Right now my checkout page takes the user off of my site and over to my shopify domain.

I am still working on a staging site so I don’t know if this is causing part of the issue.

Product: https://todaysbestdiscount.com/septictank/product/secoh-jdk-60-linear-compressor-septic-or-pond-air-aeration-pump/ (staging site)

Cart: https://todaysbestdiscount.com/septictank/cart/ (staging site)

Checkout: https://www.wastewaterpro.com/30008474/checkouts/52de092da2611ab50ded39e9fdc72c35

I did find in settings: Enable custom checkout domain
When turned on, the plugin will attempt to use the custom domain you have set within Shopify for checkout instead of myshopify.com

This is currently OFF. I’ve tried turning this ON.. but don’t see the difference.

What am I doing wrong?