Change the port in CRA's WebpackHotDevClient.

patch.js

const fs = require('fs');

// Monkey-patch react-dev-utils to get the error overlay with SSR
const pathToDevClient = 'node_modules/react-dev-utils/webpackHotDevClient.js';

// The react-scripts default
const reactScriptsPort = 3000;

// Read the dev client out of node_modules
const HotDevClient = fs.readFileSync(pathToDevClient, 'utf8');

// Switch out the socket port.
fs.writeFileSync(
  pathToDevClient,
  HotDevClient.replace('window.location.port', 3000)
);