Meeting will be limited to whatever range the NOKO airlines can reach - which isn’t far, unless Un takes a Chinese plane and the loss of face that would bring.
I’d say that despite the announced locations, the meeting will take place on the DMZ where it would be appropriate to announce the rumored end to the Korean Conflict.
Mongolia would be a thumb in the eye to China I would think.
If Fat Boy armored his aircraft the way he did his train, then range will be the end of the runway