Upgrade

I have some problems upgrading my store. Im still using 1.4.9.4 on PHP 5.6 and i wanted to upgrading opencart to the last version on PHP 7.3. Only the problems are:

The first step to upgrade my store is to upgrade it tot 1.5.x thereafter to upgrade it to the last version. I try the script upgrade-14x-to-15x.php only i recieved a "504 Gateway Time-out" error. Also i try directly the install script only to the some "504 Gateway Time-out" error.

The config.php:
<?php
// HTTP
define('HTTP_SERVER', 'http://**/');
define('HTTP_IMAGE', 'http://**/image/');
define('HTTP_ADMIN', 'http://**/admin/');

// HTTPS
define('HTTPS_SERVER', 'http://**/');
define('HTTPS_IMAGE', 'http://**/');

// DIR
define('DIR_APPLICATION', '**/public_html/opencart/catalog/');
define('DIR_SYSTEM', '/**/public_html/opencart/system/');
define('DIR_DATABASE', '/**/public_html/opencart/system/database/');
define('DIR_LANGUAGE', '/**/public_html/opencart/catalog/language/');
define('DIR_TEMPLATE', '/**/public_html/opencart/catalog/view/theme/');
define('DIR_CONFIG', '/**/public_html/opencart/system/config/');
define('DIR_IMAGE', '/**/public_html/opencart/image/');
define('DIR_CACHE', '/**/public_html/opencart/system/cache/');
define('DIR_DOWNLOAD', '/**/public_html/opencart/download/');
define('DIR_LOGS', '/**/public_html/opencart/system/logs/');

// DB
define('DB_DRIVER', 'mysql');
define('DB_HOSTNAME', 'localhost');
define('DB_USERNAME', '**');
define('DB_PASSWORD', '**');
define('DB_DATABASE', '**');
define('DB_PREFIX', 'oc_');
?>
(the asterisks are for security)
There is something wrong in accessing the DB, only i cannot the problem.

Maybe you have some suggestions?

The first step to upgrade my store is to upgrade it to 1.5.x
thereafter to upgrade it to the last version.

Well, better forget it, it won't work. The only upgradeable Code would
be the Database, and even for that, you'll need professional Support.
But everything else will be wasted time.
Ernie

For an upgrade that far back, I'd suggest contacting a developer and even looking into Qphoria's upgrade service.