Can't enter BIOS setup on M4700 after enabling Secure Boot

I wanted to reinstall Windows on an old M4700. I prepared the installation thumb drive, but when I opened the quick boot menu, I saw that Secure Boot was disabled (probably for an old Linux installation). So I enabled Secure Boot in BIOS settings and disabled legacy ROM booting. However, after that the startup behavior of the laptop changed.

After startup it now shows a black screen instead of the Dell logo. If I press F2 or F12, it just stays like that indefinitely. If I don't press anything, it loads the old Windows installation, which works fine.

I've tried disabling fast boot in Windows and connecting an external display, neither helped. Please help me solve this issue.