Hello @nextbracket,
1. The header on your website is built by elementor, you can use Elementor’s features for responsive sizes to manage it
Please follow the steps explained in this link: https://elementor.com/help/mobile-editing/.
2. The “add to cart” button is missing, perhaps a plugin affects it.
For troubleshooting, please do the following steps and let me know the results:
0. Enable the primary theme(if you are using a child theme).
1. Head over to WP Dashboard > Plugin.
2. Deactivate all plugins.
3. Then, first, activate the Ocean Extra plugin.
4. Check your issue.
5. Then, activate your plugins one by one
6. After activation of any plugins, check your issue
Note 1: You can try troubleshooting on the staging website/or on a fresh installation.
Note 2: If you have any cache plugin or server cache(CDN / Browser Cache and Cookies and …), you need to clear its cache contents or disable them to see your changes. Also, remember to click on the regenerate all assets file and data in Elementor > Tools.
Best Regards