Plugin Not Working – Potential setup issue?

Hello, quick question! I understand this is quite the minimal plugin so there shouldn’t be a lot that can go wrong in regards to the configuration but I don’t seem to be getting any results after installing the plugin and adding the variable to my config.php.

As a sample I’m using a https://admin.retrocastle.co.uk for my CMS, and I need the frontend redirected to https://www.retrocastle.co.uk when a user is not logged in. As far as I can tell I’ve got the correct configuration but the frontend pages on ther admin domain still seem accessable. I have ensured I am logged out and have tried on multiple devices.

config.php - 
define(?'HEADLESS_MODE_CLIENT_URL',?'https://www.retrocastle.co.uk'?);

Could anyone offer any insight into where I’m going wrong with this?

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