How to use decorators inside namespace: MobX

I use MobX and React in my client-side code like this:

<script src="/js/react.min.js"></script>
<script src="/js/react-dom.min.js"></script>
<script src="/js/mobx.umd.js"></script>
<script src="/js/mobx-react/index.min.js"></script>

Now When I want to actually use MobX like below, it doesn't work, i.e. my client-side user-interface doesn't react to changes of observable variables:

const action=window.mobx.action
const observable=window.mobx.observable

class Store {

    @observable isLoggedIn=false;
    @action setIsLoggedIn=(tf)=>{this.isLoggedIn=tf}
}

I wonder how I can actually use "window.mobx.action" and "window.mobx.observable" with ES6.

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Previously I was using MobX with NPM like below, and it was working fine:

import { action, observable } from 'mobx'

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