If the fan is running but its not heating on fan assist setting then its going to be a faulty fan heater element.
If you empty the oven and remove the back panel inside the oven there you will see the fan blade. Around the fan is the heater element. This is the heater for the fan assist setting. You will need to replace the fan heater and that will cure the fault from there and get it working again.
As the convection oven uses different elements thats why its heating but as you see the convection is a slow heating setting.
Just wondering how you got on with this as the question is still open. Do you still need help on this?
If so then just get back to me. But if not then if you could remember to rate the answer that would be great.