I am not sure if I am reading the issue incorrectly, but I use bluetooth & rouvy app to calibrate my magnus trainer.
I have to make sure my gears are in big ring and on a middle gear to get enough resistance to reach 18-22mph. It takes me very little effort.
When I first purchased my magnus it took 2 minutes at 18-22mph and I attempted on the small ring, and almost collapsed from exhaustion!