This link may help. Note the following from that site. And for what it's worth. I have a new Inspiron 5575 and my M.2 drive has never been detected in the BIOS. But it boots from the M.2 drive fine and works in Windows 10 without a problem.
NVMe SSDs do not appear within the BIOS until Windows creates the system partition with the EFI Boot Sector. Your M.2 SSD contains UEFI driver information within the firmware. By disabling the CSM module Windows will read and utilize the M.2-specific UEFI driver.
There have been several reports on this forum of the 970 Evo not working in some Dell systems, even on systems where the 960 Evo works fine (and also some confirmed reports of the 970 Evo working fine in other Dell systems). For the systems that don't work with the 970, I haven't seen any fixes found or even a confirmed underlying cause identified. If you have another system that can install the 970 Evo into even temporarily, you could try updating its firmware if there's an update, but otherwise you might be better off just returning it and trying to get a 960 Evo. I haven't seen any issues with that SSD not working, as long as it's installed in a system with an M.2 slot that actually supports NVMe SSDs, of course.