Reflector Director uses mDNS technology to discover your computer running Reflector Teacher. If you are unable to see Reflector Teacher from the Reflector Director app, this is most likely because your iPhone or iPad is not on the same network or subnet as the computer running Reflector Teacher. It's also possible that mDNS traffic is not allowed on your network.
Ensure both of your devices are on the same network and try relaunching Reflector Teacher and Reflector Director.
If mDNS or network limitations prevent pairing, you can also use a Quick Connect Code to pair Reflector Teacher to Reflector Director.