First post here...please be gentle if I ask something stupid

I just bought a Model D after about an year of using a cheaper rower, but I’m mainly a runner so I clearly don’t have technique and muscles, but good cardiovascular system.
I always keep an eye on the heart rate monitor as I’m used to train by heart rate in running, and I read the UT2 and UT1 bands require low spm, which makes sense. I have no problem in rowng at 18-20 spm, but I’m struggling in increasing the spm for harder workouts. In fact, what I usually end up doing is keeping the rate down, around 20-22 but pushing even more (hopefully with the right technique)... I really can’t see myself rowing at 30+ spm :-O
Again I’m reading that a > 5 power/stroke ratio is good, and mine is usually around 5.40 when I row slowly, but everything seems to fall apart when I increase the rate, with ratio going down a lot.
Is this a problem or is it simply lack of muscle power? As long as this is not wrong technique that can lead to injury I’d be fine.