HP LaserJet 400 M401dw is ALWAYS offline and will not print

My HP LaserJet 400 M401dw is ALWAYS "offline" and will not print.

It will not print from any device, whether over the WiFi or cabled connection.

HP website advice does nothing. Power cycling does nothing.

Occasionally (maybe every few months) when I turn on the printer just hopping it might work this time, it will start printing something that is in the queue. But there's no predicting whether that will happen or not.

When it does print, there are no problems at all. How can I get the printer to know it is online, and to work again?