Due to the airfoil of the trainer wing, it will tend to rise as you increase in speed. The good part of that is that it means the wing has a lot of lift which is good for a trainer.

If it were me, I'd find a throttle setting where I am comfortable flying and trim the plane to fly level there.

If you slow down or speed up for short periods, compensate with the elevator to keep it level. If you decide to speed up or slow down for much longer periods of time then maybe trim it for level flight at the new setting.