I've been having issues with DeX on the Dell G5 LT running Windows 10 Pro - my desktop works fine, the Surface 6 Pro works fine and yet the Dell G5 - WOW! Spent a couple of hours to resolve the issue in a Samsung Chat session to end with no solution except to take the phone to a Samsung service center. Long and short, uninstalled DeX on LT then removed USB drivers and the software associated with them, DID NOT reconnect phone yet, installed latest DeX software for Windows 10, then hooked the phone in and voila! The phone's "Start Now" is back and it launches the DeX on the LT.
Uninstall both of the Samsung Mobile USB drivers and include deleting the driver software. Then go to C:\Program Files (x86) and delete the Samsung folder after uninstalling the application. Download a new copy of the Samsung DeX software and install. Plug phone into USB or thunderbolt port and you should get prompted on the phone to "Start Now". Your computer should be back up and running.