The WWAN slot was not tested by Dell with an SSD so no, there will not be a BIOS update to add such support. The Latitude 5420 only supports one SSD =One M.2 2230, Gen 3 PCIe x4 NVMe, Class 35 SSD (slot 1)One M.2 2280, Gen 3 PCIe x4 NVMe, Classs 40 SSD (slot 1)One M.2 2280, Gen 3 PCIe x4 NVMe, Class 40 SSD, Self-encrypting drive (slot 1)
Hi, I used wwan slot in latitude 5490 with this m2 ssd card and it works normally but now my laptop changes to latest latitude 5420 and it seems it doesn't recognize the card in the wwan slot. the card can be recognized on slot 1 though. Any chance WWAN slot can be used for M2 SSD card with future bios upgrade? Thanks for reply.  
Latitude 5420 WWAN for M2 SSD