There were some rare odd Dells that didn't really do USB keyboards. Try PS2.
I have had great success in this forum over the years - I repair computers for seniors.
Have a Dell Optiplex which does not respond in any way to the mouse of keyboard - both are usb. I have connected known good mice, kb - both wired and wireless. Each time the computer indicates it found new hardware - prepares then indicates it is ready - but nothing. I have seen a few things on the internet but they require accessing safemode which I cannot do - since F8 and the like cannot be inputted. The wired mouse (optical) glows red for about 6 secs then nothing. Model number is 9010 - nice unit.