In the routes.js file, I tried writing a prerender function using the fetch from isomorphic-unfetch, but I always see a build error in the XDN console when the prerendering starts.
Figured it out.
Looks like you need to use the node-fetch library when you need to use fetch in the prerender function.
I ran into this issue as well. The theory was in the serverless environment, perhaps the module is confused on the context it is being called from. The error message lead me to believe it thought it was in the browser environment. node-fetch was also the solution for me.