This plugin can redirect non users or not signed in users to any external or internal url.

Unnikrishnan S

During development phase you can use this plugin instead of showing \”Under Construction\” page. This plugin will redirect all visitors to a specified url and allow developers or admin to login via wp-admin panel.
Advantages of using this plugin from Under Construction Plugin 
1> Your old site is still accessible when your new site is in construction state 
2> Admin or developer can login using site-url/wp-login.php 
3> Easy to configure 
4> Can even redirect to external urls

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