An excercise you can do (on the trainer), is without any resistance at all. Just stand up, without pedaling, cranks horizontal, weight distributed between your feet and your hands on the hoods. Start by just standing, then slowly putting more weight on one foot and back to balance, then on the other foot. Don’t try to rotate, just control going back and forth more on your feet. Try to keep your upper body controlled, and never take all weight from a foot.
Goal is of course to slowly extend the movement till you can do slow rotations. This teaches your muscles to keep control, and excercises the neuromuscular timing, both regarding the moment your single leg needs to switch from pushing to controlling the “landing” of your push at the bottom, and regarding the timing of the switch between your legs.
The excercise is pretty heavy on your quads and your knees. Especially if you already have some knee problems use extreme caution, though the excersise can also be used to strenghten the knee - just keep control.
You can slowly up the cadence and the power, but always keep control, especially when raising the cadence.