The IB-DK2106-C can only do 30Hz with 4K and that is absolutely not recommended, scrolling at 30Hz is a torture for the eyes, 60Hz is at least necessary for it to be good in the long run. In general, you can say about these docks that if a VGA connection is also included, then it's usually over with 4K and 60Hz.

The docking solution can do 2 x 4K with 60Hz:

i-tec USB 3.0 / USB-C Dual 4K Dock (USB C)