Problem with in-app and GTM

Hi guys! I'm having trouble since the past week trying to set up the in-app functionality. My process is stuck at stage 2 of the initial configuration when you hace to choose your channels. The snippet is correctly installed (I can see it is fired on every page in debug mode) but Customer.io doesn't recognize it. I tried this 2 snippets:

1- This one is from the documentation page 

 !function(){var analytics=window.analytics=window.analytics||[];if(!analytics.initialize)if(analytics.invoked)window.console&&console.error&&console.error("Snippet included twice.");else{analytics.invoked=!0;analytics.methods=["trackSubmit","trackClick","trackLink","trackForm","pageview","identify","reset","group","track","ready","alias","debug","page","once","off","on","addSourceMiddleware","addIntegrationMiddleware","setAnonymousId","addDestinationMiddleware"];analytics.factory=function(e){return function(){var t=Array.prototype.slice.call(arguments);t.unshift(e);analytics.push(t);return analytics}};for(var e=0;e<analytics.methods.length;e++){var key=analytics.methods[e];analytics[key]=analytics.factory(key)}analytics.load=function(key,e){var t=document.createElement("script");t.type="text/javascript";t.async=!0;t.src="https://cdp.customer.io/v1/analytics-js/snippet/" + key + "/analytics.min.js";var n=document.getElementsByTagName("script")[0];n.parentNode.insertBefore(t,n);analytics._writeKey=key;analytics._loadOptions=e};analytics.SNIPPET_VERSION="4.15.3";
 analytics.load(
     "YOUR_API_KEY",
     { 
         "integrations": {
             "Customer.io In-App Plugin": { 
                 siteId: "YOUR_SITE_ID"
             }
         }
     }
 );
 analytics.page();
 }}();

2- And this one is from the sources page

<script>
  !function(){var analytics=window.analytics=window.analytics||[];if(!analytics.initialize)if(analytics.invoked)window.console&&console.error&&console.error("Snippet included twice.");else{analytics.invoked=!0;analytics.methods=["trackSubmit","trackClick","trackLink","trackForm","pageview","identify","reset","group","track","ready","alias","debug","page","once","off","on","addSourceMiddleware","addIntegrationMiddleware","setAnonymousId","addDestinationMiddleware"];analytics.factory=function(e){return function(){var t=Array.prototype.slice.call(arguments);t.unshift(e);analytics.push(t);return analytics}};for(var e=0;e<analytics.methods.length;e++){var key=analytics.methods[e];analytics[key]=analytics.factory(key)}analytics.load=function(key,e){var t=document.createElement("script");t.type="text/javascript";t.async=!0;t.src="https://cdp-eu.customer.io/v1/analytics-js/snippet/" + key + "/analytics.min.js";var n=document.getElementsByTagName("script")[0];n.parentNode.insertBefore(t,n);analytics._writeKey=key;analytics._loadOptions=e};analytics.SNIPPET_VERSION="4.15.3";
    analytics.load("MY API KEY");
    analytics.page();
  }}();
</script>

Had anyone a similar problem? I don't know what else to try.