I have the following allIcons.svg file hosted on external-domain and the definition of symbol id#icon-svg-cta looks something like this:
<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" height="0" style="position:absolute" width="0">
<symbol id="my-svg-cta" viewBox="0 0 64 48" xmlns="http://www.w3.org/2000/svg">
<g>
<fill-code>
</g>
</symbol>
</svg>
and below is how my HTML looks like:
<svg class="my-svg-cta">
<use xlink:href="https://<external-domain>/allIcons.svg#my-svg-cta"></use>
</svg>
If I use the file internally it works fine but when hosted externally Icon doesnt render on any of the browsers (Chrome/FF).