I know this thread is over a year old, but I actually got the same notebook from a friend and I'm reusing it as as NAS, and I was facing the same problem.
My first attempt was unplugging ac, battery, and bios battery (at least 24h) but it didn't resolve anything.
Then I discovered why: UEFI and FastBoot are enabled, so your keyboard-interactions won't work @ startup. So what is starting up? The Windows BootManager. Then I tried following: When you insert a windows installation media (DVD/USB) it WILL start this one up.
When you get so far, try a FirmwareRepair (not necessarily) because /at least for me/ after inserting the Installation Media (!mportant that it is from Windows) I could enter the BIOS again, then changing from UEFI to Legacy/FastBoot-Disabled
Hope this helps anyone who's facing the same problem