I haven’t tested this solution thoroughly, so please check if it works for you.

Edit the wp-config.php file and place the following:

define('WP_ADMIN_DIR', 'admin');
define('ADMIN_COOKIE_PATH', SITECOOKIEPATH . WP_ADMIN_DIR);

Create a mu-plugins directory:

mkdir wp-content/mu-plugins

Create a file inside it and place the following code. Let’s name this file admin-rewrite.php

<?php
add_filter( 'site_url',  'wpadmin_filter', 10, 3 );
function wpadmin_filter( $url, $path, $orig_scheme ) {
        return preg_replace( "/(wp-admin)/", WP_ADMIN_DIR, $url, 1 );
}

add_action( 'login_form', 'redirect_wp_admin' );

function redirect_wp_admin(){
        $redirect_to = $_SERVER['REQUEST_URI'];

        if ( count( $_REQUEST ) > 0 && array_key_exists( 'redirect_to', $_REQUEST ) ) {
                $check_wp_admin = stristr( $_REQUEST['redirect_to'], 'wp-admin' );
                if( $check_wp_admin ) {
                        wp_redirect( home_url() );
                        exit;
                }
        }
}

Edit the Nginx virtual host file and place the following:

location ~* /admin/ {
    rewrite ^/admin/(.*) /wp-admin/$1 last;
}

Do a configtest and reload.