Make sure that your sensors are on and broadcasting. Open the CABLE Config app and connect to your CABLE.
In the ANT+ (IN) page, select your power meter/pedals for the Bike Power (PWR) sensor and select your trainer for Controllable Fitness Equipment (FE-C). Save your sensors.
In the ANT+ <-> BLE Assignments page, under FTMS select both PWR and FE-C. Save your assignments. By selecting both PWR and FE-C this will allow your power data to come from power meter/pedals instead of your trainer.
In the BLE (OUT) page, verify that your workout data is coming through. Completely close out of the CABLE Config app.
In the Zwift App
In the Zwift app (or your desired fitness app), select Power Source and select your CABLE. For Controllable, select CABLE.
If you have any other sensors saved with your CABLE, select them as well during your set up.