They have no way to help with North Korea, like they do with Iran. North Korea is a client state of China, and only China has a say in what they do.
Not true - according to Trump himself.
President says ‘Russia is not helping’ U.S. deal with Kim Jong Un.
North Korean state was created in Soviet times with Stalin himself installing Kim Il Sung back in the 1940s. So it is technically part of the Soviet sphere of influence, which Putin is hoping to maintain for Russia in this new century.