Rendering HTML files in an Express/React/React Router application

Hello everyone. Never posted here about a code problem, but I am really stuck 😀
I have an code editor application. I can open files, edit them and save them. At one point, I want to preview the html files in a new tab in order to see the changes I have made. Now, these are full page html, like so:

<link … />
<script src=”…" />

I can get the HTML content but I can’t find a way to correctly load the HTML in a new tab.
I tried to use a The problem is that I get a localhost:3000/filePath and the HTML would be rendered inside the React application markup, which is not what I want.
I tried to use some iframe but that failed miserably.
I believe, and I may be wrong, that I need to bypass the react-router directive to use response.renderFile from Express. How would I manage that?
So, make a post request with the file path, open a new tab and render from express the HTML content. Right?
Any tips? 🙂