Check the displays settings, you should be able to alter the externals settings.
What can happen is the setting is too high a setting. With the external connected, close the lid (Clamshell Mode) in single display it can achieve 6016x3384 (6K) at 60 Hz. If that works then you know the internal is shutting down to put all of the needed resources to the external. Otherwise in Mirror or Extended desktop the max is 2560x1600 at 227 ppi.
In addition, the internal also supports scaled resolutions of 1680 by 1050, 1440 by 900, and 1024 by 640.