And I've found reason. My laptop supports only HBR2. I will need something with HBR3 to make it work. [URL]#Dell%20System%20Models%20That%20Support%20HBR3
Hello, Recently I got in my hands S3422DW + WD19TB.  I have above connected to: P3418HW and everyting runs on Inspiron 7577 with TB3 For now P34 is connected via HDMI and S34 - DP. I cannot set S34 to use 100Hz. Max is 60Hz, which is wierd. I've double checked on: [URL] I'm only not sure which version of DP cable I'm using. Other than this - what could be the reason for that limitation? All drivers updated and dock has newest firmware.  Thanks for help.   
Inspiron 7577, WD19TB, S3422DW, no 100Hz