2nd drive install issues with Dell 9550 (p56f001)

I was hoping someone could provide me with their experience when installing a 2nd hard drive on their laptop.

My laptop came with the NVMe SSD (256GB if that is relevant) and the smaller battery so that there was room for a 2.5" drive.

I am attempting to install a 2.5" SSD.  I have the cage, grommets, and interposer.  The interposer seemed to connect correctly but the SSD isn't being recognized.

I have verified that the SSD works fine (I put it in an external USB enclosure and tested it), so there is no issue there.

I feel like it is one of three issues:

1. The interposer is bad (or not the correct one).
2. There is a BIOS setting that needs to be changed (currently set to RAID On, as it was shipped.  Changing it prevents Windows from loading)
3. There is an issue with the motherboard preventing it from working correctly.

I'm hoping or (2), but would be happy with (1), I just wish there was a way to test it.  There are several options available on Amazon that come with the interposer.  If anyone has a recommendation I'd be happy to take it.

Thanks in advance.