Unlike the MultiPHP Manager provided by cPanel, CloudLinux's PHP Selector does not allow you to configure a different version of PHP per addon-domains or subdomains.
While it is possible to get this to work, this functionality is not directly provided via the interface and instead will require some manual tasks.
CloudLinux currently has 2 different documented methods on how to achieve this.
Method 1: Using suPHP
Method 2: Using mod_lsapi