You can only use ACF’s ajax functions if you extend ACF’s JS. And you can only get something back if you build the PHP actions that return what you want. I have some examples of doing this here https://github.com/Hube2/acf-dynamic-ajax-select-example. If you want to do ajax without extending ACF’s code then you’ll need to go with just jQuery and ignore the acf functions, but you’ll still need to build the admin actions to get something back. I would suggest starting here https://codex.wordpress.org/AJAX_in_Plugins
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