Use full resolution of Dell Monitor on MacBook Pro

dell u3818dw (4k) monitor
On my macbook pro (13", 2016, 10.13.1) i am using a dell u3818dw monitor. Original resolution is 3840x1600 but my mac only runs at 2560x1440.

When i bought the monitor 2 months ago everything was fine. Only today things changed.

question: how to configure my mac to use the full resolution?

<Betreff vom Admin bearbeitet>

question: how to configure my mac to use the full resolution?