Hi,
Sorry for the late answer.
As far as I can see, the script loaded from your site directly:
Request URL: https://faircustomer.ch/wp-content/cache/busting/1/gtm-5029bae01a070c541a0dbfa15f293a7a.js
This HTML tag is added before the Google Global Site Tag (not GTM) in your site:
<script data-no-minify=”1″ async src=”https://faircustomer.ch/wp-content/cache/busting/1/gtm-5029bae01a070c541a0dbfa15f293a7a.js”></script>
I do not know where this script is comming from but it is definitely not GTM4WP.
Inside the above JS file it seems to me more like a Global Site Tag library and not a Google Tag Manager container. There are similarities since Global Site Tag (often called gtag.js) is based on GTM technology but limited only to Google tracking codes.
If you have a dedicated Google Analytics plugin active on your site, try to deactivate it, clear your cache and check whether this file is still loaded. If not, you know that is is actually part of another plugin.