RE: 2nd drive install issues with Dell 9550 (p56f001)
Even with RAID mode enabled, both the BIOS and Windows would be able to see the second drive. That option is there to force SATA traffic through the Intel Rapid Storage controller to allow for Intel Smart Response if you were using a 2.5" spinning drive and a small (32GB) SSD cache module in your M.2 slot. But until you enable that option, RAID mode treats all drive as independent and makes them all visible. So unfortunately, that would not account for your issue.
If you ever did want to change that BIOS setting to AHCI though, you might be able to get a bit more performance out of your NVMe SSD. The reason these systems ship in RAID mode is because it's the most compatible configuration, so it's easier for the factory. RAID mode enables the caching setup I just described for people who ordered their system that way, AND it allows Windows 7 to run on NVMe drives. Windows 7 doesn't have native support for NVMe, but RAID mode uses the Intel controller to mask that from the OS, so as long as you use the Intel RST driver, you can run Win7 on an NVMe drive -- and the XPS 15's sister Precision system can be ordered from the factory with Win7. But on Win8 and up where native NVMe support is available, bypassing that controller might get you some extra performance, especially if you have something like a Samsung retail SSD where you would be able to use Samsung's NVMe driver rather than the default Microsoft driver, which some users have found significantly improves performance.