I have two Dex Pads and Two Samsung S9+, my phone doesn't work on either pad but my wife works on both. I have the latest updates from my carrier (Verizon).
1: Phone doesn't show charging while docked and I am using the supplied cables from Samsung Dex Pad box.
2: I see USB connect/disconnect from on the phone when trying to connect to the Dex Pad.
3: I can get Dex to work on an after market USB-C/HDMI cable so I know the Dex program is working.
I have followed the website troubleshooting help section to no avail.