Minor Wordpress bug fix

In short: I'm trying to get my navigation menu to change to a secondary menu when the user is logged in. I've achieved this via a piece of code found online, however the code changes ALL menus on the site to inherit the values of the main menu.

The code I used is below, and doesn't seem to reference a specific menu - which i suspect is the issue, but I have no idea how to implement a solution.

https://www.wpbeginner.com/wp-themes/how-to-show-different-menus-to-logged-in-users-in-wordpress/

Obviously I can share log ins / FTP details as necessary once the job is assigned, but my site is https://music4retail.co.uk and I'm running the Phlox pro theme, which unfortunately hijacks the native Wordpress menu controls and renders a lot of the dynamic menu plugins useless.