Your access logs will show all requests (traffic) to your site, including those that resulted in 404s.
I suspect that your 404 page is not calling your function, so the request is not tracked.
You should be able to get your access log from your hosting place, via the cPanel (assuming you are not self-hosted). But most of those requests, I’d suspect, are not valid visitors.
You could also change your function to provide more info from the CURL request. See this page on the curl_getinfo function: https://www.php.net/manual/en/function.curl-getinfo.php .