Did you know that Pyongyang has ocean access on both coasts? Il sung built a huge tidal wall on the east coast to exclude salt water, reclaim crop land, raise the river level so ocean going cargo ships could reach Pyongyang and impress his dad. Kim Nam did the same thing on the west coast with the Nampo dam. Combined with some canals and dredging they connected the rivers so large cargo ships and even submerged subs can go from coast to coast without going around the RoK. These systems also connect to several rather large and deep reservoirs.
Their first missile launching sub is a modified Sang-o which is pretty small and can easily reach those reservoirs, their new sub probably can as well but it would be noticed, not that it really matters. .
So how to you reliably track and sink a sub at the bottom of an inland lake before it launches?