You can create a Elementor template and then assign it into the Footer slide up.

To get started, please follow the steps below:
- Create your template and copy the it’s shortcode, navigate to Dashboard > Ruby Templates. To create a template, you can refer to this documentation: Ruby Templates

- To add Ad block, you can refer to this documentation: Adding Adverts to the Elementor page

- After creating the Ad block, copy the shortcode of this template.

- Navigate to Theme Options > Ads & Slide Up > Footer Slide Up
- Click to enable Footer Slide Up Section option

- Input a ruby template shortcode into Slide Up Template Shortcode input

- Save your changes.