main.js
const { useReactiveHook } from '@apollo/client';
const { isLoggedInVar } from './cache'
import App from 'pages/app';
import Login from 'pages/login'

const Main = () => {
  const isLoggedIn = useReactiveHook(isLoggedInVar);
  return isLoggedIn ? <App /> : <Login />
}

Directly use the useReactiveHook

isLoggedInVar is a Reactive Variable