They updated the BIOS?

yes 1.3.4 new bios 3 improvements, can find here  www.dell.com/.../advanced

I have 1.3.4.  I bought 2 Corsair Dominator DDR4 2666 CL5. Should have check the forum first. Oh well... It only run at 2133.

The 5675 can use RAM up to 2400 MHz, according to the specs.

downloads.dell.com/.../inspiron-5675-gaming-desktop_reference guide_en-us.pdf

If you still have old RAM installed that's 2133 MHz, your new 2666 MHz RAM will run at that slower speed. Otherwise, if you only have the new RAM installed, itshould run at 2400 MHz.

Supported RAM configs are: 4GB, 8GB, 16GB, 24GB, and 32GB.

That was  what I expect. but both are new RAM, CMD16GX4M2A266C15. It only runs on  2133 with C15.

Do you have the latest chipset drivers?

Guess it's possible your new Corsair RAM isn't fully compatible with this PC, even though the specs may be correct.  

Maybe you should contact Corsair Tech Support...?

Thanks RoHe. I checked with Corsair. XMP is needed for 2400 or 2666. other it will default to 2133. since XMP is a Intel thing, 2133 is all it will do with AMD processor.

For those with XMP memory limited to 2133 you can get Thaiphoon Burner that allows you to edit the SPD EEPROM of your memory to add JEDEC 2400 (and higher) support.  As a warning, I have only used Thaiphoon Burner to add JEDEC 1600 support to DDR3 modules (never on an Inspiron 5675 or DDR4 modules).  This tool will void your memory warranty (backup your original SPD EEPROM) and could make it unusable.

The Thaiphoon Burner website only list support to ddr3 memory!!

Did you look at the official website?

http://www.softnology.biz/

Edit: You can download a demo version that does work with DDR4 and can check if you can write to the SPD EEPROM of your memory (most DDR4 is likely read only for the JEDEC portion).  Note Windows Defender detects a virus but I believe it is a false positive.