Can't enter BIOS after changing from Secure/UEFI to Legacy

I had to go into the BIOS and change the type from Secure Boot / UEFI to Legacy Boot in order to get the firmware program for my SSD to run from a removable USB drive. No big deal, I figured after the firmware is updated, I'll go in to change it back to Secure / UEFI, right?  Wrong.

So, hours later now, I cannot enter the BIOS at all. During the POST screen, I see the "Press F2 for Setup / F12 for Boot Options" and I even notice that the "F2 for Setup" become highlighted/selected after holding the F2 key down. But, instead of going into the BIOS, it goes to a DOS menu in which you can either select the OS or Windows Memory Diagnostic.  

I ended up doing a clean install of Win 10, and disabled fast boot. That didn't help. Then I thought installing Win 7 would fix it. Nope. Same deal; can't get into BIOS.  I did some googling and found one suggestion to press Alt+F2 and all combinations thereof including F8, ESC, DEL etc - no luck.  Another suggestion I tried was removing the CMOS battery / unplugging the SSD / laptop battery in order to force it to a hold POST screen.  What happened then after pressing F2 was lots of beeping and a Boot from PXE menu came up (Bizarre).

Any other suggestions?  I can use Windows but Win 10 doesn't have full functionality without UEFI / Secure Boot enabled. And I'd hate not being able to access the BIOS or boot menu forever.  I hope some Dell expert can help me get in to the dang BIOS.  I might add that the reason for me updating my SSD's firmware is crashing issues related to the Win 10 Anniversary update... separate complex issue lol...

Appreciate the help. -Dave

Inspiron 11 3137 - Win 10 x64 - 128GB Transcend SSD - 8GB G.SKILL RAM