Worked fine before.
I am buying expensive workstation hardware so I can use them for a long time, I have the full expectation to keep using this machine for another few years.
Dell.... you are working on an updated driver, right?
Yeah, I just checked support.dell.com. You definitely shouldn't wait on Dell to release an updated driver. Windows 10 isn't even listed as an option in that system's Drivers and Downloads section. It is after all pretty old at this point. But as I said above, I've found that drivers direct from the GPU vendor work just fine on discrete GPUs. With integrated GPUs you sometimes need drivers specific to your system (or you at least need to go through extra steps to force a generic GPU vendor driver to install), but that wouldn't apply here.
If the updated driver from NVIDIA doesn't work, it would help to explain what setup you're trying to run (is the built-in panel one of the displays are you using docking station, if so what model docking station, etc....) and also explain what you see in Display Settings and/or NVIDIA Control Panel, i.e. is the third panel shown but can't be enabled or not detected at all, etc.