Why Omi mirror doesn’t find the device that I want to cast to?
Please make sure that your Roku device and Mac are connected to the same Wi-Fi and under same subnet. If you still have problems discovering your device, here are some troubleshooting tips that can help:
Restart router. Just unplug it from the power supply, wait 30 seconds, then plug it back in.
Restart device. Unplug it from the power supply for a full reboot. Please disconnect it from its power supply for at least 1 minute, then put it back in. Powering off your TV is NOT ENOUGH to do a full reboot.
For more possible network problems, please check How to setup network to cast my Mac to TV wirelessly?