how find motherboard suited for these Ram ?

Just to reiterate Kimputer's comment you cannot mix UDIMMs and RDIMMs on any motherboard so nothing takes both at the same time.

i do not want to mix the sticks - i only want to know which ram is suited for which mobo
so - plse - update your posts, i tyhink you all have good answers, but list me a couple in the budget range under 250$
( preferably one cheap, and one higher priced )

For the RDIMM ASrock EPYCD8-2T seems about the cheapest you'll get, a bit over $250 but nowhere near $2K.

What's wrong with my URL? It was ALL under 250 euros already?

andyalder - what is the RDimm ?
Kimputer - nothing is wrong - but can you point out a couple  plse?

thank you both 

kimputer, how do you define the chipset needed ?  that would help me a lot in the future 

Intel provides all the info. Per CPU architecture, there's a set of chipsets that work with the CPU.
After that, just find the mobo's from the manufacturer's website, as they usually put the chipset in the name.
In your case, it was DDR4-2666. Find the CPU where that's the limit. CPUs that go higher will have your memory as the bottleneck, so it's not worth your money.
After I found it's the 8th/9th gen Intel https://www.intel.com/content/www/us/en/products/docs/processors/core/9th-gen-core-desktop-brief.html
Then find the compatible chipsets next: https://en.wikipedia.org/wiki/Coffee_Lake (Chipsets has it's own chapter, all listed there)
On any shopping site, just use "ASUS B365" as the search keywords to get your compatible mobos, continue searching if you want a more or less expensive chipset (from the set you already found earlier).

Thank you Kimputer - that will help me out

Kimputer  -  i was looking at the  Asus Prime H310M-K board, but the KCP ram is not in the QVL list
are you sure it is compatible ?

It costs someone about $1000 to do all the testing and paperwork to get on a qualified/supported list such as Asus QVL list. There are hundreds if not thousands of DDR4 sticks so you are not going to find every make/model in their list. Any DDR4 UDIMM within reason should work with that board.

Correct, consumer boards with incompatibility problems with the big brands (Kingston, Samsung, Hynix, etc) are like winning the lottery. If you stick to the correct specs and brands, it will never be a problem.
Incompatibility starts with the server boards.

ok guys,  and thanks
i hope i do not have to instruct my chipdevil to send bad vibes - if it does not work