The next version of Steam Link for iOS and Apple TV is now available as a beta build.

You can opt into the beta on TestFlight by clicking on the link here:
https://testflight.apple.com/join/Cye7Gfyy

New in this build:

• Added the ability to specify exact values for streaming bitrate, framerate, and resolution in the streaming settings
  
• Fixed a hang in streaming settings if the microphone isn't available