Figured it out. Removed the onboard battery from the mainboard for a bit, and was able to get the AHCI option to stick. I'm more concerned why this worked even after restoring factory defaults. I will take the win. 
Greetings, Brand new Dell 3910 with 256GB NVMe. BIOS came with RAID mode enabled. I want to switch to AHCI mode for various reasons, most importantly I want to install Windows 10 fresh from USB, and if in RAID mode requires the RST driver, and I'm not doing that. However, the BIOS is refusing to allow me to do this. The option is there, and when I toggle it I'm presented with the option to apply the changes, but they don't stick. Upgraded BIOS to newest version via wifi. No fix Ideas?
3910 can't enable AHCI mode in BIOS