iframe content isn’t loaded correctly

We have included content from an external supplier in an iframe on our website. It’s on a different domain and we have no access. This worked for years. The supplier changed to a responsive webdesign. Now we have problems to show the content in IE 11 correctly. Other browsers are ok.
the parent website: http://www.piag.info/verwaltung/mietangebote/
the external website: http://www.homegate.ch/mieten/alle-mietinserate/trefferliste?a=x217&incsubs=1&l=neutral

Advanced iframe version: 7.1.2

Do we need advance iframe pro or is it a css problem?
Any help appreciated!