Gonna resolve this. My USB-C cable was not the correct kind of cable. I did also need to enable "transfer power via USB-C" in the UEFI BIOS. (I didn't have the enabled before and it worked, but it seems appropriate for what I was trying to do.) Also, for whatever reason, my dongle also began to work. I did download and update with all of the latest drivers for my 7210 2-in-1. I'd say that helped for Windows 11 to get working appropriately. So, updated drives and a new USB-C cable capable of transferring video was the key to this device working with my ThinkVision M14.
Not sure what to say here. Windows 10 worked fine. I was given the option to upgrade and ... well, I shouldn't have. Windows 11 fails to detect any externally connected monitor that worked in Windows 10. ThinkVision M14 is my standby, but it neither supplies the power nor seems like it understands the DisplayPort language that's being spoken to it. The fact that it worked before I upgraded tells me there's an issue of drivers or ... that Windows 11 is truly a step backwards. I have also hooked up my Ultrawide monitor via the Displayport and Dell USB-C port replication dongle (Model DA300u) and ... pretty much nothing works on that, either. No power passthrough. Complaints of the dongle itself not receiving enough power. I checked the UEFI BIOS and it found some port controls... one was to disable everything but display and device connectivity. I switched to that. I also found the setting to enable powering and charging other devices from the laptop, and I enabled that. No joy after reboot. All of the same "lack of power" and the occasional, "we can't understand your newly connected monitor". Again, keep in mind, this all worked before Windows 11. Did I jump the gun before this device was truly ready? I'd love to hear people's thoughts, feelings, and, especially, solutions to these issues.
Upgraded my 7210 2-in-1 to Windows 11. No more multi-monitor. Not USB-C, nor 
dongled.