wp_remote_get – cURL error 28 – only on same domain

By default, WordPress set the timeout value for cURL in wp-includes/class-wp-http-curl.php to 5 seconds and the same value is also set to HTTP requests in wp-includes/class-http.php that is a newer class for making HTTP requests that can use also cURL if it’s present in the server.

In your case, 5 seconds was not enough to perform the WP REST call, but it was enough to perform the dummy call. Generally, in terms of doing an HTTP request 5 seconds should be enough in most cases. But it seems in your WP REST endpoint you are doing too much work that need more than 5 seconds.

You can increase the timeout value to any value (15 seconds for example) by the following filter.

function custom_http_request_timeout( ) {
    return 15;
}
add_filter( 'http_request_timeout', 'custom_http_request_timeout' );