No bios, no boot after disabling secure boot

Device: Dell Inspiron 15R SE 7520, with own SSD. Dual boot windows 10 & Ubuntu 18.04. No battery anymore (physically removed)

Situation: I wanted to live-boot a new Linux distro from USB. Insert the stick, boot to boot menu (f12). USB stick doesn't show up next to the other options, but there is an option to disable secure boot (or at least I think that was what it said). I chose that option since I thought I remembered I had similar issues before that were solved by temporarily disabling secure boot.

The problem: after disabling secure boot, pressed reboot. Power went off, but when it went on again, nothing happened except for the fan spinning up temporarily and the power button LED going on. No bios screen, no windows boot, not even the backlight going on.

What I tried: power cycle multiple times. I tried to reset the bios by removing the CMOS battery and disconnecting AC power, pushing the power button for 30 seconds, leaving the battery out for half an hour or so. Reconnect the AC power, try to boot then, 5 bios beeps for missing cmos battery. When I put the battery in, still 5 beeps for missing bios battery. Disconnect AC, reconnect, click power button. System powers on (power button led on and ventilator blowing). After 1 second powers down by itself and after one second up again. Still nothing.

I also tried all f buttons, delete button. Control+escape during plugging in the AC does not work since the laptop is from before the Dell bios recovery tool era.

I'm at a loss for what to next. Does anyone else have a clue how one setting in the bios can mess everything up and how to fix it?