U2719DC, doesn't show the menu USB-C Prioritization?

@gaga Try this:

1. Disconnect the DP cable currently connected to the DP Out port on the U2719DC, i.e. the one feeding the U2715H.
2. Reset the U2719DC to factory defaults using the onscreen menu.
3. Do you see USB-C Prioritization now?  If so, enable MST and then set USB-C Prioritization to High Resolution before reconnecting the DP Out cable.

Hi thanks for the tips, but it dosen't work....:-( 

@gaga  Well while helping someone else, I found a couple of interesting items.  As of this writing, if you go to the support.dell.com page for the U2719DC here, and click the link to access the User Guide PDF, you get taken to this PDF.   As of this writing, the bottom of Page 2 of that PDF says "Rev A04", and Page 42 does mention the USB-C Prioritization option, as you say.  But when I Googled "U2719DC Manual", I found a direct PDF link to this document.  It's also hosted on the downloads.dell.com server, but under a different folder.  As of this writing, Page 2 of that PDF says "Rev A02", and that PDF doesn't mention the USB-C Prioritization option at all.  It SHOULD appear on Page 40 of that version of the manual (since there are 2 pages in the A04 manual dedicated to "Using the OSD Lock Function", which doesn't exist at all in the A02 version), but as you can see, the A02 version just mentions MST and then moves down to Reset Display, whereas the A04 version of the manual describes USB-C Prioritization between MST and Reset Display.

So now I'm wondering if an earlier hardware revision of the U2719DC didn't have the USB-C Prioritization option and it was instead added later.  In that case, perhaps even a firmware update to an older hardware revision of the U2719DC wouldn't add it.  I admit that would still be strange, though.  I wouldn't expect an option like that to be added as a hardware revision to an existing product model.  I would normally expect that to be part of a newer, completely model if it required a hardware change.  But even if a hardware revision is what happened here, it would be hugely disappointing and confusing for users to read the latest revision of the documentation for this display and see this option mentioned, while not finding it on their own displays.  The only other explanation I can think of is that this option was always supposed to be there and was simply omitted from the earlier documentation revision by accident.  That would normally be the most plausible explanation, but of course that wouldn't explain why you can't actually see this option on your own display.  Somehow that doesn't feel like a coincidence.

I'm going to bring this up with a Dell Support contact who's been helpful investigating issues in the past to see if he can get some information internally and provide a clear answer here.

Thanks for your help, I will appreciate to use my second screen at full resolution with one cable from my PC as it was expected on the manual.

For dell support, my S/N CN-04W8HW-Ws200-95R-A6UW-AD5....

Hello. I am awaiting your response. Do reach out with the requested information so that we can assist you further. ^MP

Hello. I have not heard back from you yet. Do reach out to us at your convenience so that we can assist you, until then we shall be temporarily archiving the case at our end. ^MP