If you disabled Fast Startup in Windows, that is not the same as Fast Boot in BIOS, if Fast Boot is enabled in BIOS, that may be blocking you accessing BIOS.
Click the power button on your Start menu.
Press the Shift Key and click Restart
Your PC will boot into the Windows Recovery Environment
Go to Troubleshoot - Advanced Options - UEFI Settings.
Your system should then restart again and boot into BIOS.
If not, what is the exact make and model number of your system, the method to boot into BIOS varies from PC to PC.