Tacx flux s bad resistance...

I had Elite suito T and my MTB on it and everything works fine when I drove power mode. I bought Tacx flux S and I Cannot drove any more with MTB because cannot reach desired power on low cadence!

Now I ride with a road bike and I have to change gears ( it,s very annoying)  to achieve desired power. Does anyone have such problems? How to solve the problem?

Thanks !!

Sory for my bad english