Have you confirmed that your PXE environment boots with Secure Boot on other systems?  If so, try restoring your BIOS to factory defaults just in case something has gone wrong with the Secure Boot keystore on the affected systems.

First I would start by verifying that BIOS settings are configured properly to point to Legacy boot. If that does not help you narrowing down the issue, take a look at general steps when it comes to troubleshooting PXE issues: https://jprudente.wordpress.com/2014/02/18/troubleshooting-pxe-booting/https://tgw.onl/digitalocean/ https://tgw.onl/siteground/ https://tgw.onl/ipage/

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@arso96 wrote:
First I would start by verifying that BIOS settings are configured properly to point to Legacy boot. If that does not help you narrowing down the issue, take a look at general steps when it comes to troubleshooting PXE issues: https://jprudente.wordpress.com/2014/02/18/troubleshooting-pxe-booting/

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The OP specifically wants to boot with Secure Boot enabled, and having Secure Boot enabled prevents enabling Legacy boot....

And setting to Legacy works- but I"m not sure what that proves as Secure Boot won't work with Legacy anyway. 
UEFI works with secure boot off. 
It works with ALL other models of DELL Laptops we have   3160, 7450, 3380, 7240

Im not sure if you made any progress with this, but I just got a few hundred 3190's and am running into the same issue.  I did notice that in the bios the check boxes for secure boot were on, but at the boot menu it clearly said that secure boot was off (right out of the box, before I even touch them). I let a one of them build completely and from inside windows confirmed that secure boot was indeed off.

Eventually (think 2 or 3 days of pulling my hair out), I went into the expert key settings and checked the check box for custom keys.  Rebooted.  Went back into expert key settings and unchecked the box for custom keys and acknowledged the message that the keys would be reset to the defaults.  After that I was able to PXE boot.  The boot menu also clearly said that secure boot was on now. 

I'm thinking they have something wrong in the secure boot keys from the factory, my next stop is to try to export them and see if I can figure out what is going on.

Some of the units I have unboxed are on bios 1.1.0, some are on 1.2.1 and I have the first few I got working up 1.3.0.  They go back to broken if I reset the bios to factory/default settings.

in the bios go to secure boot - expert key management- check the box Enable custom mode and the uncheck it.

Exit and pxe boot again.

It should work.