I imagine pretty easily. They likely don’t have to find it, nothing gets dug in North Korea without US watching it via satellite. Chinas air defenses likely don’t exist in North Korea.
I have no doubt the US has plans to cut them (and probably anyone else you can imagine) off in the event of war.
It's probably of higher intelligence value to let them connect and intercept everything than it is to cut them off and not know what's going on. If you look at their border with China, it's only tens of meters from fairly populated areas, so setting up high bandwidth microwave links wouldn't be hard. Also bombing a sovereign nation is an act of war and comes with consequences.
All the “international sovereignty” responses are humorous to anyone whose paid attention to the last twenty years of the American military.