Dell has been building machines that are more picky about RAM since Haswell with DDR3L, so you need to stick to using quality RAM in anything newer than Ivy Bridge if you aren’t working with original modules (usually Samsung or OEM Micron). I usually recommend Crucial (Micron) or Kington in the Dells since it seems to be the most compatible. They aren’t as picky as HP (the BIOS will reject modules with a RAM error at POST if the timing is inconsistent on Elite and Z), but Dell has clamped down on the tolerances with the newer computers. The days of throwing parts bin RAM are mostly over if it uses DDR3L or newer.

It sounds like you may have spilled something on the motherboard since you mentioned IPA - if you’ve already tried to repair it at the board level, then usually that means there’s serious damage somewhere that cannot be repaired. Might be time for a new motherboard or laptop; the board are expensive.

Look at it this way, especially on the RAM tolerances Dell has pushed on: I miss the days when you can get whatever is cheap on eBay as long as it works with 1.5V DDR3, but they had to do it because the voltage tolerance between DDR3 and DDR3L was reduced by .2V; DDR4 bumps that to .3V, so it was unavoidable. You can’t get away with accepting anything that’s in the slot when you went from 1.5V down to 1.2V! Even then, it was better to use good RAM, but it usually worked if you got it cheap and were okay with the genuine risk of a no POST error.