CTG is correct.  OP is also correct that WWAN slot may accommodate B+M-keyed ssd in selected motherboard(s).  OP says (s)he has XPS desktop and not XPS laptop.  XPS desktop PC mobo is not equipped with WWAN technology, which is primarily used in laptop or tablet/iPad for on the road wireless access via cellular data. M.2 connectors on the host are called "sockets." Each socket has a unique mechanical key, and modules are not interchangeable between sockets. According to PCI Express M.2 Specifications Rev. 1.1, the sockets are distinguished as follows: An M.2 B+M key SSD may support SATA bus or PCIe x2 bus. If the SSD only supports SATA bus (so-called M.2 SATA SSD), its speed will not exceed 600 MB/s. If the SSD supports PCIe x2 bus, its speed will be around 1000 MB/s. WWAN + Utilize cellular technology to securely transfer data or connect to the Internet WLAN+ Most computers come with WLAN technology built in+ commonly known as Wi-Fi
  This never occurred to me until a quick search turned up a YT video of someone installing a 512Gb SSD in the M.2 slot of a laptop meant for the wireless card. In my XPS desktops, I've never had the need to use wireless.  Seems too good to be true that I could pull out the M.2 wireless card and drop in an M.2 SATA 2242 SSD.    
XPS 8940, M.2 SATA SSD in the M.2 WWAN (wireless slot)