ACF only stores the ID of the attachment, so if you’re accessing the meta_key directly that’s all you’ll get.
I did a quick look at the information on this plugin and there’s no information on how to display images other than the featured post image using this plugin.
If you can find out if this plugin will display other images in custom fields and what information needs to be stored in the postmeta table you can create an acf/save_post action to create the field it needs.
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