P2720DC, XPS 15 9550, not working over USB Type-C

@TW69  I just wanted to add a follow-up note from my post above if it takes any of the sting out of the answer I provided in the thread I linked.  Over there I basically explained that the best you can do is to use a USB-C to DisplayPort cable.  Obviously that means you lose the ability to carry power and USB data over a single cable from the display, but if it helps, neither of those are going to be significant losses for you given that you're on an XPS 15, for the following reasons:

In terms of power, Dell USB-C displays don't supply 130W over USB-C, which is what the XPS 15 requires for proper functioning.  The most they currently supply is 90W, but the P2720DC only supplies 65W, i.e. only HALF what the XPS 15 is designed to have available.  As a result, even if you're connected to a USB-C display via USB-C in order to receive power from the display, you'd need to keep the system's own power adapter connected for optimal performance.  Otherwise, if you have your XPS 15 operate solely from that undersized power source, it will respond by reducing battery charging speeds and throttling CPU and GPU performance.  And with only 65W, the reductions in charging speed and performance would be substantial.

As for USB, in order for your system to run 4K 60 Hz over a "native" USB-C connection (i.e. USB-C to USB-C), the USB data rate has to be limited to USB 2.0.  The reason is that the XPS 15 only supports DisplayPort 1.2/HBR2 over USB-C, which incidentally is also still true of most systems on the market today.  In order to run video and USB 3.x data rates simultaneously over USB-C, two of the four high speed lanes in the USB-C cable need to be dedicated to USB 3.x, leaving only two for video -- and on an HBR2 system, that isn't enough for 4K 60 Hz.  You instead need to use all four lanes for video, but then you only get USB 2.0, which can be utilized because USB-C connectors have separate pins dedicated to USB 2.0, and therefore running that doesn't interfere with the four high speed lanes.  The only way to run 4K 60 Hz and USB 3.x simultaneously over USB-C is with a system that supports DisplayPort 1.3/1.4 (HBR3) over USB-C, but those systems are still fairly rare today.  (There's also Thunderbolt 3, which can run dual 4K 60 Hz and USB 3.x simultaneously, but that isn't "normal" USB-C.)

So while the answer I gave in the linked thread might be disappointing, at the end of the day you're only out the cost of having to buy a USB-C to DisplayPort cable, and you've lost the option to connect USB 2.0 peripherals to the display without having to run another cable from the display to your laptop.  But if you DO get the USB-C to DP cable and choose to ALSO run a USB 3.0 cable from the display to your laptop, you'd be able to get USB 3.x that way.

Hopefully this helps!

@TW69  See my posts about this in this thread.  It explains the underlying problem and offers a workaround, but there's no proper solution, unfortunately.