pfg’s answer is about using a different port to do the Let’s Encrypt validation. And if that’s what you want, yeah, that post is right you can’t do that. But I think you already HAVE a perfectly nice certificate for your name synodins.com, and you would just like to ALSO use that certificate for this server on port 8002. That IS possible. But, as you noticed it is not working right, so you need help figuring out why.
First observation, that port isn’t speaking SSL / TLS at all, it’s speaking plain unencrypted HTTP. You need to configure the server answering on port 8002 to offer TLS encryption. A “nodejs” person might have more insight than me.