Only visible in athor:
Make sure the desired page to be shown in the iframe contains this script-tag:
<script src="https://www2.telenet.be/content/dam/www-telenet-be/templated/modules/resizable-iframe/src/iframeResizer.contentWindow.min.js"></script>

Info window:

Page in iframe: https://comm.int.telenet.be/optiext/optiextension.dll?ID=9%2BD7lcdIqsWL%2BGw04NIP%2BK5upRGZtYEnLOd60UN%2BDI4y%2BYN%2ByF4GPVcgURDWzCTz7bY8p4CQS4VT9t