ExpressJS Server Side Rendering renders blank page

[sheptang]

I'm creating a web application with many pages as components and it uses ExpressJS framework on the NodeJS backend. I want to implement SSR to improve performance (since it's a relatively small project) and be ready for the best SEO practices.

However, even though my server setup looks just fine (no errors on console from nodemon or React), when I visit the server on the browser, I see a blank screen, where the #root is not filled with the App component.
I'm not sure if whatever I'm doing is wrong, because the examples I could find online did not match exactly with what I'm dealing with.

The React render on localhost:3000 works perfectly. But, the SSR on localhost:3001 shows me a blank screen, with the empty index.html loaded (#root div is empty).

Here is the server.js for ExpressJS:
https://pastebin.com/uFZApGHc

The App component:
https://pastebin.com/p93UPFz5

Express package.json:
https://pastebin.com/pz9KijwR

React package.json:
https://pastebin.com/1xP4PzJP

Project structure:

If there is any other information needed, please tell me and I'll include them. Any ideas or contributions are appreciated.