I have found a workaround solution to this problem.
While Rouvy is already running and I am on a climb, I open Wahoo app on my iPhone and I adjust wheel circumference. Lower circumference = lower resistance in Rouvy. Power readings and speed in Rouvy stay accurate, only the gearing range needed for the same output power changes.
I hope someone from Rouvy will read this. A simple "virtual gears adjustment" would solve this problem.