I hadn't really thought about it before, but I believe North Korea has an 840 mile border with China, which is not hostile to them.

So I don't understand how sanctions by other more distant countries could meaningfully affect them if China did not choose to acquiesce.

Either China is oppressing them, or it's an internal problem.

https://en.wikipedia.org/wiki/China%E2%80%93North_Korea_bord...