Third monitor not working on Precision 7550 and WD19DCS

I have Dell Precision 7550 connected to WD19DCS dock. Graphic card is Intel  UHD. Laptop LCD screen is disabled in Intel Command Center.

Until yesterday I had a three external monitors setup and working. DP and HDMI in dock and mini DP in laptop port. It all worked fine until I disconnected laptop from docking. After returning I can get signal from dock or from PC, but not from both. If I disconnect both HDMI and DP cables from dock, then mini DP from laptop starts working. All monitors and cables are good.

Does anyone know how to enable the mini DP port on laptop?