One of the highest-rated USB-C, 4K monitors that you can buy right now also comes from LG, the 27-inch ‘ 27UD88-W’. *Available in December from and Apple Stores.
MAC PRO 2016 INPUT FOR MAC MONITOR MAC OS
You might want to look for an EDID Override for Dell monitors on Mac OS X. I don't know what OS you have on your MacBook, or the year, so can only give general suggestion. Under "About this Mac" then "System report", the Dell should show up as a display. choose either 30 or 60 hz, whatever resolution you can. See if the cable is HDMI 2.0 and HDCP 2.2 compliant.Under system preferences, then displays, go to scaled. On the Dell, you should be able to switch and prioritize the inputs.
Yes, check the adapter (if you're using one) and/or the cable. I also presume you're not trying to get 4k out of the monitor-you just want a darn picture. I presume you are using an adapter to connect the Dell Monitor to your MacBook. However-Dell is known to not play nicely with Macs of any species. I used to use a Dell 27" monitor with my Mac Pro desktop, so I know it basically works.