Press F12 and check the console. I am getting this message:
Your clien’ts site is secure (https), but the source of the embedded iframe is not.
BTW, iframes are notoriously difficult to make responsive. There’s this:
https://benmarshall.me/responsive-iframes/
which is older, but works (or at least it did the last time I tried it). But just because the iframe itself is responsive does not mean that the content will be all that responsive, or even be usable on smaller devices.