Precision 5570 no longer sends 4K signal through WD19 TBS Thunderbolt port

Hi all,

I have a Precision Workstation 5570. I purposefully purchased the Dell WD 19 TBS because it supports both Apple Mac M1 Pro and the Dell Precision at full 4 K resolution when having one cable utilizing the Thunderbolt port to one Dell 27" 4K monitor and then using the display port to the 2nd Dell 27" 4K monitor. 

For the longest time, I could swap Mac and Dell without issues and the Mac and Dell could drive 4K out of the thunderbolt port through to the docking station without issue.

Occassionally, if the Mac battery power is low, I would have to connect power and TB connector as the WD 19TBS could not supply enough juice to power and accept the signal.

However, recently, the Thunderbolt port out the back to the monitor stopped working for the Dell only. I know have to connect the HDMI in separately, disconnect the TB cable for the Dell. If I put in the Mac, I have to use the TB port and Displayport. 

What could be the issue? How can I make the Thunderport work again for display purposes so I don't have to swap cables everytime I swap systems?

Pavel