db-config.php

<?php

// MASTER machine
$wpdb->add_database(array(
	'host'     => DB_HOST,     
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => DB_NAME,
	'write'    => 1,
	'read'     => 0,
));

// REST READ-ONLY
$wpdb->add_database(array(
	'host'     => '192.168.1.9',   
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => DB_NAME,
	'write'    => 0,
	'read'     => 1,
	'dataset'  => 'rest',
	'timeout'  => 0.4,
));



$wpdb->add_callback( 'my_db_callback' );

function my_db_callback( $query, $wpdb ) {
	if (  defined( 'REST_REQUEST' ) && REST_REQUEST ){
		return 'rest';
	}
}