Feature image does not display properly on mobile

The problem is this max-width value that should be 100% instaed of fix pixel.

.post-img img {
	max-width:1080px;
	height:auto;
}

But I can’t say for sure, it’s not normal for theme nowaday that don’t use responsive image CSS. Maybe it’s the theme settings that not configured correctly, or maybe it’s the case of user makes changes to theme file incorrectly.

Since the theme you are using is a commercial theme, so it’s best to go through its own support channel.

But meanwhile this piece of CSS will fix it (I only tested it in my browser web dev tool).

.post-img img { max-width: 100%; }

The code must be used via theme’s Custom CSS option, or via Custom CSS plugin for it to work (because it needs to be loaded after theme’s main CSS).