Keyboard is not detected bu the bios

I have a dell e7240 computer. All of a sudden the keyboard stopped working. I did everything I could do. By the way, I'm an electronics technician. No success. I bought a brand new keyboard. This didn't work either. I updated the bios. It didn't work that way either. So the bios doesn't detect the keyboard. I couldn't find the wiring diagram of this computer and I don't know who is in charge of the keyboard. I bought another matherboard. I installed the new keyboard on it and it still doesn't work. I can't explain what's going on. I've fixed a lot of computers, but I'd never come across a problem like this. All the information on the internet about this problem starts with images showing instructions to recover the keyboard, a comical situation, by pressing f2 or f5. Well, how to press these two keys if the keyboard doesn't work at all. It's just silly! Do you have any ideas?