DELL WD15 DOCK with Macbook Pro 2018 - max resolution issue
The limitation is the Dell WD15 Dock only supports HDMI 1.4a (see specifications), which only supports 3840 x 1600 at 24p, and the dock does not support HDCP (content protection). Your Mac wants to run at 60p unless it is playing a movie, in which case Netflix wants HDCP. Count yourself lucky that your 2017 Mojave Mac will let you use 3840 x 1600 at all.
The fix is to use DisplayPort. Get yourself a mini-DisplayPort to DisplayPort cable (around $10) and use that to connect your dock to your display and you should be able to get the full 3840 x 1600 at 60 Hz support.