How did you get Windows 11 Pro installed on a Legacy Bios? I know turning off secure boot on UEFI is possible with Windows 11.
On my PC I can switch from UEFI to Legacy but I can't boot into Windows 11 with Legacy.
Try the boot menu key, not the BIOS key, which on my PC allows me to switch easily from Legacy to UEFI (Secure or unsecure). https://www.disk-image.com/faq-bootmenu.htm