I have the same question about similar Area-51 R4 system with an Intel CPU (i9-7940X). Would anyone report if it's upgradeable to 128GB RAM and if yes which modules you have tested with it. I know of course that officially Area-51 R4 only supports up to 64GB RAM, but the CPU is good up to 128GB -- wonder if anyone had success with four 32GB DIMM's.
UPDATE: I tried 128GB kit with my Area-51 R4 (i9-7940X model), but it failed to boot - blank screen, beeping sound :-(.
Specifically, I tried this kit: "CORSAIR Vengeance LPX 128GB (4 x 32GB) 288-Pin DDR4 SDRAM DDR4 2666 (PC4 21300) Desktop Memory Model CMK128GX4M4A2666C16".
Please let me know if anyone can find 128GB RAM kit that works (if it's possible at all).
Finally, if anyone from Dell is reading this, it would be great if you could release BIOS update to enable 128GB RAM on these machines, if it's technically possible on these specific machines (CPU and chipset already support that). I would not be surprised, if many users would even pay up to few hundred USD to be able to install 128GB RAM on these otherwise excellent machines.
Usually when the extra RAM is not supported, you would expect it will show 128GB installed, and 64 usable. Like, the PC will still boot, you just can't use the extra RAM.
I haven't tried 2 sticks actually, so I don't know if that works or not. Even if it did, it would just be purely academic since I would like to increase the RAM amount above the current 64GB (in 4 stock sticks).
Really like the machine, just wish I could have 128GB RAM, as many other X299-based motherboards allow.
Actually the reason I asked was to see if there was a compatibility issue with the RAM you purchased because what you described is not what would typically happen if the only issue is the PC does not support that much RAM.
If the issue is 128 is not supported it would still boot fine, you would just be limited to using the max supported.
My primary hypothesis has been that it just doesn't support higher density sticks (required to have 128GB in 4 sticks), but you are right, it IS possible that the specific sticks that I have are incompatible. I unfortunately can't sample all possible 32GB DDR4 sticks on the market, so I was hoping someone had success with 128GB on an X299 Area 51 R4 so I could buy the same ones.