Finally resolved 5290 KB issue. Solution:  Bought same keyboard on Ebay ($23 like new).  Plugged it in, worked instantly. Conclusion:  Dell bios flash ruined the keyboard controller rendered complete failure.Dell had no response to fix my keyboard.  They don't care even though it is clearly their fault.My previous experience with HP on hardware issue is they will replace it if it is their issue.Lessons learned on Dell support.  Never again.
I recently updated the BIOS on my Latitude 5290 2-in-1 to the latest ver 1.21.0.  My detachable keyboard is no longer detected.  No response on keys and touchpad and lights no longer lit up.  I remember when the BIOS updated, it also flashed a keyboard section and it said it flashed successfully.  I have re-flashed back to previous BIOS version, KB still not working.  And I flashed back to the latest BIOS, still not working.   I have checked that it is not in tablet mode, does not have keyboard turned off.   The BIOS HW boot check goes through everything ok but did not detect any keyboard when I touch the keys.  I have re-seated the KB many times, no contact issue. I knew it was unwise to upgrade the BIOS.  I was gonna upgrade to Win 11 and wanted to upgrade the BIOS before doing that.   Now without the attachable KB, it is really unusable.   Of course this is no longer under warranty.  Any help is appreciated to fix the keyboard.
Latitude 5290 keyboard not working after BIOS upgrade