Polyfills are the answer, where support is provided for features that don’t exist in the browser, such as with Internet Explorer. There are a number of fetch polyfills as evinced in this article:
I’d start with the most basic one, that being the whatwg fetch polyfill. Go to their releases page and obtain the fetch.umd.js file.
Then include that script in your HTML page which polyfills (only if needed) fetch, providing support for it when browsers don’t know how to do fetch.
<script src="js/fetch.umd.js"></script>