Ultimate Site Maintenance
Using this extension you can put Magento 2 sites into maintenance mode from the admin, and set a custom maintenance mode message as well. Instead of having to use FTP or SSH to put your Magento 2 sites into maintenance mode, you can use this extension to create a custom maintenance mode message, including using a WYSIWYG editor, set the timer to keep it in maintenance mode for different store views and also specify IP addresses those are allowed to access the frontend.
