We are experiencing an issue with the Add to Cart functionality on mobile device

Hello,

01. Please deactivate all the 3rd party plugins and activate only theme-required plugins on the site, and then check the issue. I am sure your issue will be solved. Then, activate the 3rd party plugins one by one and check which plugin is creating the issue for you.

Otherwise, if the issue still exists, then keep the 3rd party plugins deactivated and share the WP admin login details of your site so I can check and give you a possible solution.
02. A database connection error is generally not related to the theme itself. In most cases, it is caused by server-related issues.

For more details follow this guide and configure the options:
https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-error-establishing-a-database-connection-in-wordpress/

Best Regards