Is your site displaying something like
404 error ipfs resolve -r /ipns/your-site-1234.on.fleek.co/app: no link named "app" under ipfshash ?
The current problem is that IPFS gateway doesn’t support rewrite policy, so it’s not possible to redirect 404 resources to index.html (that’s usual flow). So if the path after / doesn’t exist or folder doesn’t contain index.html, it ends up with this error. We will add this to our gateway soon.
In the meantime, to solve this you need to use a hash router in your react app or some static site generators (like Gatsby or Hugo) that creates the folder structure for your routes.