Thank you for the information. As you may already know the modern event calendar plug-in is a bundled extension in our team. The bundled extensions are added by a contract between us and the plug-in developer. We include the code and the plug-in the extension section, so our customers can make use of those plug-ins without having a purchase key.
Our knowledge regarding the extensions are limited naturally, as we are not the developers of the plug-ins and do not know the ins and outs of the code, but we do our best to be as helpful as possible within the limitations of our knowledge about the plugins.
For more information about the Bundled versions of the extensions kindly read the last part of the article below:
I personally checked the documentation of the plugin and found out that the payment gateway is not supported out of the box in the plugin. Instead, they use the Woocommerce plugin payment Gateway. For more detailed information please check the payment gateway section of the document below:
I'm afraid for deeper support and official support services of the plug-in, you will need to purchase the plug-in to have a valid purchase key to get the support from the plug-in developers themselves.
Thank you for your understanding.