First, make sure to install/enable the MailChimp for WordPress plugin. This plugin has built-in styles made in Rey, but you could also use a different type of newsletter and use its public form shortcode.
Now we’ll need to create the actual popup. There are various ways of doing this, but i prefer to just edit the Footer global section, add a new Section, transform it into a Modal section and adjust its settings eg: https://d.pr/i/kMWWWz . This will load this popup in any page.
To make the modal automatically open, choose an option in Auto Popup setting – either on Page scroll, on page load (after specific time) or on Exit intent. Select the one you prefer the most. You can also disable nagging (to stop showing up).
An alternative is to access Rey theme > Global Sections and create a new Global section as Generic and edit it with Elementor, and create a modal section (like in the scenario above). Lastly you’d need to make sure to access Customizer > General > Global sections and publish this global section for example After Site container.