[Plugin: Checkfront Booking Plugin] Inline mode not working

I’m developing a theme and I want it to support inline mode for the plugin shortcode. Currently only frames mode works, but it looks ugly. When I switch to inline mode, the only thing the shortcode generates is the string “Searching Availability…”

In the plugin settings page, it mentions that inline mode is only compatible with certain themes, but it gives no more information and no hint as to what the key difference might be. Google provides no answers either.

Can anyone tell me how to make my theme compatible with inline mode?

https://www.ads-software.com/extend/plugins/checkfront-wp-booking/