Make sure the port your application is listening to is the same port URL is mapped to.

The thing is, it runs/works just fine at first but when I restart, it doesn't and my app is listening to the right port

Can you elaborate on “at first” and “restart”?

At this point I’d try making your app listen to an arbitrary port (eg, 3000) and change the port on your URL to match that as well.

I mean I deployed the codes from my github repo and it worked fine, I didn't even changed anything I just restarted it and it stopped working

Try what I suggested above.

Also, make sure there aren’t any errors in the deployment logs.

I'll somehow fix it, your solution is crashing the server but can you tell, how can I install java on nodejs server?

You’d need to use a custom Dockerfile to do that.