There are several ways to fix mobile usability issues in WordPress:

1. Use a responsive theme: A responsive theme automatically adjusts to different screen sizes, including mobile devices. This ensures that your website looks and functions properly on mobile devices.
2. Enable mobile-friendly features: Some WordPress themes come with features such as mobile-friendly menus, responsive sliders, and responsive galleries. These features help to improve the mobile user experience.
3. Use a mobile plugin: There are several WordPress plugins that can help to improve mobile usability. Some popular options include WPtouch, Mobile Detector, and WPTouch Mobile Plugin. These plugins allow you to create a separate mobile version of your website that is optimized for mobile devices.
4. Optimize images: Large images can slow down the loading speed of your website on mobile devices. Make sure to optimize your images by compressing them and using the appropriate image size.
5. Test your website: Use a mobile testing tool such as Google’s Mobile-Friendly Test or Mobile Test from WebPageTest to check the mobile usability of your website. This will help you identify any issues and fix them.