Discourse (actually Rails) sends a X-Frame-Options header that tells browsers that the site does not wish to be embedded in an iframe, hence all modern browsers will simply render a white square. You can see this in e.g. Chrome’s dev console which says
Refused to display 'http://archy.no-ip.org:82/' in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN'.
The only way to “fix” this is to hack Discourse yourself: move the EmbedController's before_filter and ensure_embeddable method into the main ApplicationController and set the embeddable host site setting. But, while it can be done, you’ll be on extremely unsupported terrain…