Windows IE 11 supports Vp8/ Vp9 while iOS Safari 12 supports the H264 codec.
Due to this video codec mismatch, you will not be able to establish video connectivity between a peer on Win IE 11 using the Free Plugin and a peer on iOS Safari 12.
However, our commercial plugins come with H264 support. You can achieve the desired connectivity with either our Standard or Custom Plugins. Please contact sales at firstname.lastname@example.org for more information about our commercial plugins.