Dug into this further and our iOS devices aren't receiving any of the handshake messages sent by the Session Frontend (or any other UDP messages for that matter) because multicast is failing to be configured for UdpMessageMulticastSocket:
1. Create a new IOS project
2. Launch on an IOS device which is connected to the same LAN as the editor
3. Open Window | Developer Tools | Session Frontend
The session does not appear under "My Sessions"
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-65365 in the post.