Thanks @lhsi
The reason for the 404 on the index of the backend site is there is no index.html as explained in [Support Guide] I’ve deployed my site but I still see "Page not found”.
I see there is a serverless function node-fetch that connects to a Heroku deploy that seems to only return some basic JSON but cannot guess how you are using it.
The third thing is outlined in the support guide shared by @luke as the backend is an Express.js app and uses a listener which doesn’t work on Netlify. It is possible to run Express.js apps as outlined in the following post