Press F12 and check the console. I am getting this message:
Mixed Content: The page at ‘https://www.harmonyhomeimprovements.co.uk/doors/’ was loaded over HTTPS, but requested an insecure resource ‘http://firstname.lastname@example.org’. This request has been blocked; the content must be served over HTTPS.
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:
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.