The DPRK has uranium mines and reactors capable of producing plutonium. They got help from the Soviets back in the 60s and 70s.
The science is all done. Everyone who wants to know how to make a bomb (or a rocket) knows or can find out online pretty easily. So it isn't a lack of smart people that's holding the DPRK or any other country back.
Atom bombs are easy. Good (efficient, compact, cheap) atom bombs are hard. To make an atom bomb all you need is a critical mass of fissile material in two chunks. Shoot one chunk into the other and you'll get a Hiroshima sized explosion.
Rockets are pretty hard. That actually is rocket science.
All of it takes wealth. You need materials, manufacturing, lots and lots of power, etc. and all that stuff costs money.