Just that. Yes, I went back to the loco and reset the control system to ZWS (it defaults to KWS). I have tried cycling the MCB, starting train line power, looked all over the Steuerwagen for a control system selector switch: nothing. Just a maddening red "HS Aus" warning, and no power. (Yes, I did unlock the brake key!)
Solved it. If anyone else runs into this: you have to switch the loco from KWS to ZWS* before you energize the cab car with the reverser-- or cycle the reverser off and forward again. That initiates the connection. *The Dosto cab cars are set up for running with modern ZWS; the 218 has to be manually changed from the older KWS protocol the n-Wagen use.
This is why it's so maddening that when you take over a train in service, all that stuff should be set up by default. Also why we need a Run 8 style pop up box that allows you to shortcut the start up procedures.