All you can do is get a refurbished replacement that will have the latest firmware pre-installed on it.To receive assistance from Dell Chat support, they need to verify the warranty status and ownership. Click the DELL-Cares username. On the right, click, "Send this user a private message". Share the private P3421W Service Tag and all of your troubleshooting with them.
I am trying to update my P3421W with the latest firmware (M2B207), but when I run the file it says that the Model No and Firmware is not detected (I have tried using both USB-C and USB-B uplink cables). I have previously updated the firmware of my monitor to M2B205 with no issues whatsoever.  I note that another user is trying to update their G3223D and encountering exactly the same issue that I am.  Is there an issue with all of the recently released monitor firmware files?
P3421W, firmware M2B207 not detecting monitor