Hard to say exactly how to accomplish this. We don’t interact directly with anything in the theme template files, so ultimately it’s a theme issue more than a CPTUI issue.
That said, very likely the post type archive is loading a different template than what is otherwise being used, OR there is logic in like the archive.php file that isn’t working out to do two columns for you. Perhaps say it’s checking if viewing just a category archive, or an archive for the “post” post type. You’d need to check with the support staff for the theme in question, if it has one, to know for sure.
Alternatively, if you’re using a pagebuilder and it’s powering the 2 column layout, you may need to see how to apply the same to this new one, or contact their support as I’m sure they would be able to get you on the right next step forward.
