I constantly still have problems with my Dell 4k monitor (first gen and last year's model), where it doesn't wake properly from sleep. Most of the time, I resolve it with the following.
- Switch the USB-C port from one to another - Open up my laptop from clamshell mode and switch the cables around - Unplug my monitor - Restart my computer
The nuclear option of rebooting my computer and monitor always seems to work.