The relationship field in ACF will only work with posts on the same site.
If you’re talking about another site on a multisite installation, there is a custom field add on for this https://github.com/tmconnect/acf-relationship-multisite.
If you’re talking about a completely different site, then you’d need to build a custom field type https://www.advancedcustomfields.com/resources/creating-a-new-field-type/
You could probably also do this using a simple select field and dynamically populate the choices https://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/, how you’d get those choices I can’t say.
