20 seconds isn’t long enough. You wanna go for at least 40-50 seconds per frank (longer if it’s a bigger one - I like the Nathan’s 1/4 lb. dogs and those need more than a minute in the microwave), so the fat in the dog has enough time to heat up and do its thang. Cut four or five shallow slits along one side about an inch apart, then flip it over and do the same to the other side. Put it on a paper towel, set it at the outer edge of the rotating glass plate (this is key - you want it exposed to the hotter outer edges the whole time rather than the cooler hole in the center) and turn it over about halfway through the process.
You’re not gonna get the same kind of char you’d get from cooking it in a skillet, but you should get a similar effect to briefly dipping it in a deep fryer.