I know about 4K 30Hz limitation for WD15 dock. But it's not possbile to set even 30Hz ( When I try to create custom resolution in Intel app, I get error about "maximum bandwith limit". Fail ...Sure TB16 dock is the better choice. But I already have WD15
Ok, then just to confirm, are you using either the DisplayPort or Mini-DisplayPort input on the display rather than a MiniDP to HDMI cable, for example? That display might not accept 4K 30 Hz over its HDMI input. Otherwise, have you tried another cable? The only other possibilities I can think of are either that the display might not accept a 30 Hz signal, or the dock might be faulty, because assuming you don't have any other displays connected to the WD15, it should absolutely be possible to run a single 4K 30 Hz display using the mDP output.
I just mentioned the TB16 as an fyi or possibly if you were within the return period of the WD15 and might want to upgrade. Many users do not consider 30 Hz to be an acceptable experience and/or want to use more than one external display.