When restarting DRE while in ACC, DRE takes a long time before connecting after restart.
The workaround is to pause the sim so it shows the in-car pause menu, before restarting DRE.
If you missed this step, you can either wait a few minutes, or restart the simulator session, by going back to the main menu.
If nothing happens for a while, try exiting out to ACC main menu and then start the session again.