This is essentially due to a video codec mismatch. IE11 supports Vp8/ Vp9 while iOS Safari 12 supports the H264 codec.
Our commercial plugins come with H264 support. Please contact sales at sales@temasys.io for more information about our commercial plugins.
Other articles you may be interested in: WebRTC support on iOS browsers| Getting Started with the Temasys SDK for iOS| SkylinkSDK for iOS on Github