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.telenet.be/optiext/optiextension.dll?ID=LKHLGvhqt5MfzfudMy7biJJrmSleN7FidDvrKK9ZjNNTIBhd9A3UxhxJQOa7WZYg0_VEsCkbBG0HLc