Thanks everyone for your inputs. The 503 ERROR is still occuring even though I've deactivated most, not all the plugins. I'm doing it one by one to identify the culprit, if it's a plugin.
Deactivating all plugins on the website is the best way to find out if any of them are causing issues. It is also a good idea to temporarily deactivate the custom WordPress theme and switch back to the default one. When the error is gone, you can then begin activating each one to find out which one is problematic.
If the error persists after deactivating the plugins and themes, you may wish to contact your hosting provider as they will be able to resolve the issue more effectively. If your provider is unwilling to help, it might be a good idea to find a new one.
The 503 service unavailable error occurs when your web server is unable to get a proper response from a PHP script. This PHP script could be a WordPress plugin, a theme, or a misbehaving custom code snippet.
https://www.wpbeginner.com/wp-tutorials/how-to-fix-503-service-unavailable-error-in-wordpress/