Daisy chain Dell Latitude 7390 to U2791DC to U3219Q via USB-C and DP ?

@Gral  I'll give you the technical background of why it's not working if you're curious, as well as options.

Technical background
There isn't nearly enough bandwidth to run the display setup you have via daisy chaining.  A regular USB-C connection (i.e. non-Thunderbolt) has four high speed lanes that can be allocated to carrying either video or USB 3.x data.  When all four lanes are carrying video, the Latitude 7390's USB-C port provides one full DisplayPort 1.2 interface, i.e. 4 lanes of "HBR2" video bandwidth -- but in that setup the USB-C link can only carry USB data at USB 2.0 speeds, since USB 2.0 has separate dedicated pins within the USB-C connector.  But the U2719DC actually sets up the USB-C link in a mode that supports both carrying both video and USB 3.x data.  That allows you to use the USB ports built into the display at USB 3.x speeds, but it also means you only get 2 lanes of HBR2 video bandwidth.  That's enough to run the U2719DC's QHD resolution, but not very much else.  (If you're wondering why the U2719DC even has a daisy chaining output in that case, it's because some systems support a faster HBR3 bandwidth standard that would leave enough bandwidth for a second QHD display, and also because some people might feed the U2719DC on its regular DisplayPort input instead, as I do.  In the latter case, running a second QHD display even from an HBR2 system is possible, but then you have to use a separate cable for USB data.)

But even if your USB-C link was running in a way that used all four lanes of the USB-C link for video -- in which case again you'd only get USB 2.0 speeds -- that would still only be enough bandwidth for a single 4K 60 Hz display.  That's how the U3219Q configures its USB-C link, since it needs all of the high speed lanes for video when being run from an HBR2 system.  And that's why it doesn't offer a daisy-chaining output, since there isn't enough bandwidth left for another display unless you have an HBR3 system, but those were very rare in 2019 when it launched and they're still pretty rare now.  And strangely, the U3219Q doesn't support HBR3 at all, even though the U2719DC does.  Not sure why that is.

Solution(s)
If you want to run those two displays from that system, you've got at least one option and possibly two, although only one of those will be a single-cable solution and that option is only available if your Latitude 7390 was ordered with Thunderbolt 3, which is optional on that model.  That matters because Thunderbolt 3 allows the system to run 8 lanes of HBR2 video bandwidth, i.e. twice as much as USB-C in "all video mode", which is enough to run up to dual 4K 60 Hz displays -- AND it can also run USB 3.x simultaneously.

- If you do NOT have Thunderbolt 3, your only option is to run the U3219Q via USB-C (in which case you'll only have USB 2.0 speeds, although that's fine for a keyboard and mouse) and then connect the U2719DC via HDMI.

- If you DO have Thunderbolt 3, the only way to carry enough bandwidth for both displays, power to charge the system, and USB 3.x data all over a single cable is to use a Thunderbolt 3 dock like the Dell WD19TB.

Hopefully this helps.

@Gral  glad you found my earlier post useful.  As to your new questions:

- Yes, if you have the lightning bolt, you have Thunderbolt 3.

- No, not all USB-C cables work as Thunderbolt 3 cables, in fact there's quite a bit of variation in terms of USB-C cable capabilities even BEFORE considering Thunderbolt 3 support.  The WD19TB dock I mentioned comes with a Thunderbolt 3 cable semi-permanently attached though, so if you went with that dock, you wouldn't have to buy one.  You only need a Thunderbolt 3 cable between the laptop and the dock. You would NOT need a Thunderbolt cable anywhere else unless you wanted to use some other Thunderbolt 3 peripheral through the dock, but your displays are just regular USB-C, not Thunderbolt 3.  In any case, if you're really curious about USB-C cables, I wrote a long post about the various types and all of their pitfalls here.

- In terms of ports that you can use, it depends a bit on the dock.  For example, the WD19TB has some restrictions on the display output combinations you can use when running setups with higher bandwidth requirements.  Basically, if you're running a dual display setup and at least one of them is 4K 60 Hz display, as is the case with you, then one of them has to be connected via the "upstream" Thunderbolt 3 port on the far edge of the dock (not to be confused with the regular USB-C output next to the HDMI port).  Again, you don't have to use an actual Thunderbolt 3 cable just because you're connecting to a Thunderbolt 3 port because you're not using a Thunderbolt 3 device, but the restriction has to do with how the dock allocates video bandwidth across its available video outputs.  The other display in your setup can be connected via any of the other display outputs.  It doesn't matter which of the displays you put on the Thunderbolt 3 port.  It's just that there will be enough bandwidth to run a 4K 60 Hz display available on the TB3 port, and enough bandwidth to run a second one available shared across all of the other ports, so you have to keep the two displays separated to ensure that each display has enough bandwidth.