I purchased a Dell XP 13 7390 last month. I purchased a companion Thunderbolt 3 dock from Cable Matters. This is Intel Certified and supports charging up to 100W and 40gbps. It comes with a 2.2ft Thunderbolt 3 cable.
The details are on Amazon:
Using this setup, everything runs fine. I use the 2 Thunderbolt 3 connections on the left side of the laptop that allow for charging.
I moved my setup around a bit and needed a longer Thunderbolt 3 cable, so I purchased one from Cable Matters that is a 6.6ft active Thunderbolt 3 cable, supports 100W charging and 40gbps. The details can be found here:
The problem I am having is that when I am using the longer Thunderbolt 3 cable, the thunderbolt connection randomly disconnects/reconnects. I've read much online about Dell XPS laptops disconnecting due to Killer WiFi, but I have replicated this problem even running in Airplane mode when using the longer cable.
Cable Matters has been assisting and helping me debug the last few weeks. I sent back the long cable and they gave me a replacement to try, but I still have the disconnection issues. Today Cable Matters came back and said that they tested their Dell laptops with my cable and the same model Dock and they didn't see any issues.
So right now, the suspicion is something with the laptop. I'm running all of the latest drivers and the latest BIOS as of this writing. I don't know if this is just growing pains given this is a new model (and maybe fixed with future driver/BIOS updates), or if there is something wrong with the hardware.
Just want to update this so other owners are aware of it.
On the product support page for drivers and downloads, there is a toggle for selecting "Show Drivers for Service Tag XXXXX"
If I unselect that, I see additional downloads for the XPS 13 7390 laptop. One of which is a new Intel Thunderbolt 3 firmware update that was released Jan 2, 2020.
I've ran this and updated the firmware from 41.02 to 44.02. I will update once I've done more testing.