The problem you are having is because you are using yarn build to create a production build (which you don’t really need to do in the dev-rig, you’re not pushing the rig to production), and then using yarn start, which is for running the development build.
If you read the output of the build process it specifically tells you how to serve the files in the build folder that you just created. You’re running the wrong command if you want to use a production build of the dev rig.