i5-8400 will work with Z390.  Dell does not restrict cpu to be used only on their motherboard.
Dell Inspiron 5680 i5 8400 1060 3g I want to keep my processor the i5 8400 but upgrade the motherboard too put into a different case so I can get a 3000 series card but I know that dell Sometimes make their parts so you can only use their products I was thinking a MSI z390-A pro socket type: LGA1151 can anyone please let me know if this would work?
Motherboard upgrade 5680