I replaced a failed motherboard (KWVT8) on XPS 8700 with refurbished one. No other hardware changes. BIOS is A10 and using Windows 10, formerly booting off HDD. Current boot up shows "System operating in manufacturing mode" and eventually shows "No boot filename received", "Exiting PXE ROM" and "Reboot and select proper boot device or insert boot media...".

In boot options (F12) the boot mode that shows is "Legacy, Secure boot off". I can select "Windows Boot Manager" under "UEFI Boot" and it boots okay. But if I change the boot settings in the BIOS setup to match this, save and exit, it apparently does not save as it reverts to "Legacy, Secure boot off" and fails to boot.

What should I do?