Reading true the conversation I gather that you have set your trainer as a classic trainer Like this image.
When using the classic trainer setting zwift tell you on what resistance setting to set the trainer. The reason for this is Zwift have a Power/speed curve programmed into the software, each trainer has one power/speed curve programmed. If you use a different resistance setting then Zwift wont know it and your power numbers will be wrong.
The Power/speed curve look typicaly like this: (This is a random trainer curve, I dont have your trainer curve)
The way it work is your speed sensor will tell Zwift your wheel speed, lest say 30km/h then using the curve Zwift will see that 30km/h = 350w