IS is blocking CORS requests by default. I added a new section in the blog explaining the process of adding CORS filters in Identity Server 5.10.0.

Please check the following section.

"3. Configure IS to allow our application to access the APIs."

The same information was added to the README as well.

https://github.com/brionmario/is-samples/tree/master/react-oidc#allowing-cross-origin-requests-to-the-identity-server

Hope this solves your issues. Thanks for reading the blog. Cheers!