Issues with document.getElementById

Hello,

I'm using svelte 4.2.18.

I need to integrate a library in a project that loads some div that contain a video dynamically by specifying the id of the container. The issue is that the getElementById always returns null.

The lib woks that way so I can't use <div bind:this={someVar} />

<script lang="ts">
 export let src; 
 let rmpVast: any;
 const id = "rmp-container-vast";
 let elementRMP: any;

 onMount(() => {
    // here we should be able to get the element but it fails so also the library fails to load.
    elementRMP = document.getElementById(id);
    console.log("elementRMP: ", elementRMP); // always null

    initVast();
  });

  const  initVast = () => {
    const params = {
      ...
    };
    rmpVast = new RmpVast(id, params); // 💥 this lib only works by specifying the id of the div
    rmpVast.initialize();
    rmpVast.loadAds(src);
  }
</script>

<div id="rmp-container-vast" class="rmp-container">
  <div class="rmp-content">
    <video class="rmp-video" playsinline controls>
    </video>
  </div>
</div>

I've also tried other ways that use $: elementRMP = document.getElementById(id) but that also doesn't work.

EDIT:

I've realised the thing is inside a shadowRoot

https://preview.redd.it/an2aoq71mhjd1.png?width=749&format=png&auto=webp&s=679fae65d4928d395d2119a2d79901347d71f97e