Precision 3581 - Upgraded RAM ( 2x16GB ) doesnt boot - Help me :(

Kingston Fury RAM is designed for gaming systems that allow adjusting the memory timing -- which your system does not.  

For guaranteed to work RAM, they have a configurator you can use to search for compatible RAM.  So does Crucial.

https://www.crucial.com/compatible-upgrade-for/dell/precision-3581#memory

One way or another, Fury RAM is never going to work with your system.

But my system support 4800MHz (from what the manual says) , it doesnt need to be downgraded. 

The problem is not the RAM speed -- it's the timing.  Gaming RAM is aggressively timed (i.e., cache latency).  It's designed for use in a system where you can adjust the timing -- which you cannot in the system you have.

Use a memory configurator from Kingston -- which will come up with the RAM they tested to work with your system.

https://www.kingston.com/en/memory/search/model/107505/dell-alienware-precision-3581

I bought additionally 2x16GB DDR% 4800MHz ( i bought specifically this model of Kingston - KF548S38IB-16 ( i dont if it matters but it support XMP - i didnt make any change to BIOS or to memory at all though- )

If you notice the specs for that module, the 'standard plug-n-play' memory timings (CL 38-38-38) are the same as the XMP. Which means, this module is already out-of-spec @4800MHz. The JEDEC standard line card for DDR5-4800 is CL 40 or CL 42.

For business or professional workstations, you should avoid any of these 'overclockerz" gimmick modules. You can tell them by the juvenile sounding marketing names they give, like FURY, PHAZER, HYPER, DEMON, BALLISTIX, EXTREME, REACTOR, and silly marketing like this that sounds like characters from Marvel Comics.

Your DDR5-4800 module/kit should be JEDEC standard with a CL (CAS Latency) of 40 or 42. No XMP.

I suspect some foul play from Dell.

Two new Precision 3591 Mobile workstations with 1x32GB SO-DIMMs factory installed. 
The original PN in each is SKHynix HMCG88AGBSA092N AA which is a 32GB 2Rx8 PC5-5600B-SB0-1010-XT

Initially I got a Kingston KCP556SD8-32 (32GB DDR5 5600MT/s Non-ECC Unbuffered SODIMM CL46 2RX8 1.1V 262-pin 16Gbit).  I tried it as a standalone module and as a 2nd module with the factory installed one with the same result - No POST. I tried changing slots, nothing.
I thought it's DOA so I RAM'ed it and got another identical Kingston - same issue. The factory module boots no problem but the Kingstons won't. Tried it on the 2nd Precision 3591 - the same issue. 

So I returened the Kingston modules thinking somehow it's a compatibility issue with Kingston (depspite Kingston's site saying otherwise). Then, I got a pair of SKHynix HMCG88AGBSA092N BA: 32GB 2Rx8 PC5-5600B-SB0-1010-XT  - both these modules either in standalone or as pair with each other or the factory - system won't POST.  

To me that suggests Dell is somehow black listing OEM modules. It's very unlikely that 4 different memory modules in 2 different laptops do not work despite having the correct specs, timing, etc. Considering Dell charges 300% more for the same memory module it's absurd. It screams some foul play..

@ifisher_condor​ Any update?

After installing the new memory, powering on are you allowing several minutes (up to 15 max) for DDR5 'training time'? During this period, it may appear the system is 'dead', no display.

Wee upgraded dozens of the 3571 3581 and 3591s with identical kits of 2x16GB Crucial CT2K16G56C46S5. They worked for months. Then a BIOS update nuked about half of them. Had to clear CMOS and put factory memory modules back in. 

Super bogus.