Allow external iframes local IP

I am at the beginning of migrating from on-site SharePoint to SharePoint Online.

Trying to get a page viewer to display an HTML/PHP page that is hosted locally. I understand that this will cause problems when viewing the pages when off-site, but that is ok for the requirements.

Is it possible to set Site Settings > Site Collection Administration > HTML Field Security > Allow External iFrames to accept content from internal servers, local IP (eg 192.168.1.*)?

If so, what "domain" is needed in the above settings (or otherwise) to allow these pages to display?

The URL pointing to the local page works in web browsers fine, just doesn't display on SharePoint Online.

Thanks,

Lachy