U2415 as second external monitor - wrong resolution

Good morning,

I have 1:Dell XPS9500 > 2:USB C to Dell Monitor U2421E > 3:DP connection to Dell U2415. 

It works, but, in the Windows Display Setting, the display resolution of the U2415 is only available in 1920*1080. Not in 1920*1200. 

Could you please help?

Thank you in advance.

Gerald