document.getElementById().onclick not working inside Google Tag Manager

I am using a cookie consent management service, calling it via Google Tag Manager (custom HTML tag). Their script has function CookieControl.open(); which opens a sidebar to allow users to change their preferences.

<script src="https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js"></script>
<script>
var config = {
...
...
...
};

CookieControl.load(config);

window.onload = function () {
    var cookiesExist = document.querySelector('#cookies');
    if (cookiesExist) {
        document.getElementById("cookies").onclick =
            function () {
                CookieControl.open();
            };
    }
};
</script>

In my footer, I have added a link <a id="cookies">Cookies</a> which the function in GTM is supposed to target. Unfortunately this is not working and the sidebar does not open when I click on the "Cookies" link. It does sometimes work, randomly, on Safari on the iPad. But it's not consistent. Any ideas why most of the time this doesn't work (but other times it does)?

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