You might want to check out the instructions here for recovery mode or contact Apple Support.
For recovery mode, at the last step, keep holding the power button until you see the recovery mode screen.
For DFU mode (which isn't documented in the link above), timing is really important, so it can take a few tries to get it right.
I've been charging it for the whole day with different cables, bricks, and outlets. I'm gonna leave it overnight just to be sure.
If you charged it for more than an hour already, it should be more than enough.
Nope, no signs of life at all.
Maybe there could be a hardware issue as well, for example either a power or a screen issue. When you're following these steps, you should see the Apple logo on the screen at certain points - and if you don't, it's either a hardware issue or an important part of the boot process being broken (iBoot). If it's the latter, DFU mode should still work.
Here are some detailed explanations for that, if you're interested. I'll use some technical terms here - we're on the DevForums after all 😁.
iBoot is the stage 2 bootloader that loads iOS / Recovery Mode on your iPhone. It can be changed and upgraded. I believe the Apple logo is shown during this stage.
BootROM is the stage 1 bootloader that loads iBoot or DFU.
I think DFU can't break during updates, since it's essentially Boot ROM code waiting to be recovered via USB. So if you really cannot enter it at all, it's a clue that it could be a hardware issue.
