it will show this warning message:
https://www-slmexpress-com.filesusr.com’s server IP address could not be found
See here for working with iFrames in Wix.
https://support.wix.com/en/article/using-iframes-to-display-visible-content-on-your-site
You will read on this page about the difference between using html iFrames on free Wix sites and premium Wix sites.
Code within the HTML element is displayed on your live site within a sandboxed iFrame. Using a sandbox protects visitors to Wix sites from potential side-effects of custom HTML/JS/CSS code.
Also note that on this page Wix state that if your used code comes from a third party provider, then you need to go back to the third party code provider as Wix won’t support code from a third party.
We cannot provide support for external codes that were not created or tested with Wix. If you are experiencing an issue with your code snippet, please contact the provider directly.
Although, as filesusr.com is where Wix host all their HTML bits, then it could be an issue with that server or somewhere else on their CDN.
See this other post as it seems to be working again now.
https://www.wix.com/corvid/forum/community-discussion/iframe-is-broken