Did you look at these Boot options that are listed in the Inspiron 3670 Service Manual?

Region code must be set for the drive and the optical Disk must have a UEFI bootable disk in it before you turn power on or it wont show that as an F12 option.

ANY DVD or CD must be 64 bit.

You did not specify what OS you are trying to install.

32 Bit OS of any Kind or Legacy Boot is no longer allowed.

Windows 10 64 bit DVD will boot with the Region Code Requirement.

Current 64 Bit OS DVD can be created with the media tool

Use Media creation tool to create uefi 64 bit dvd.

Requires Dual Layer 8 Gig dvd  old 4.7 Gig dvd is too small

if you dont know how to create this disk buy a dvd that you can boot from.  You dont need a key because its in the bios.

https://www.newegg.com/microsoft-windows-10-pro-64-bit-reinstall-recovery-disc-only-no-license-key-included/p/N82E16832350238

Dell only supports the OS that it shipped with

https://go.microsoft.com/fwlink/?LinkId=691209

in the case of UEFI secure boot must be turned off.

The other way to test this theory is to burn a UEFI Puppy Linux CD that wont suffer from the Region Code problem.

http://distro.ibiblio.org/puppylinux/puppy-bionic/bionicpup64/bionicpup64-8.0-uefi.iso

Thanks that good info, but I am not installing an OS, but just booting a Live Linux Mint CD.

I did check region code and they are correct.

UEFI secure boot is turned off

Linux Mint CD MUST BE 64 bit and support UEFI.

You cant use Non UEFI Version and You cant use 32 bit OS versions,

Linux Mint 20.1 Ulyssa

https://linuxmint-installation-guide.readthedocs.io/en/latest/

The easiest way to install Linux Mint is with a USB2 stick.

Burn the content of the ISO onto the DVD, not the ISO file itself. When finished, your DVD should contain directories such as boot and casper, it shouldn’t be an empty DVD containing an .iso file.

To prevent issues, burn at the lowest possible speed.

Neither Dell Nor I are supporting every distro of Linux under the sun.

The puppy CD i linked will uefi boot and it will see the system and drives etc.

Thanks again.

Come to think of it that Mint CD may be 32 bit.

I have no current intentions to install, just was giving it a test run.

Kinda odd it will boot & run with the work around I have on my first post though.

Plus I totally agree with the USB option.