The new router is slightly fussier than the old one about SSID passwords, so now my printer is offline.
The printer manual is here:
http://support.hp.com/us-en/product/HP-Deskjet-3050A-e-All-in-One-Printer-series---J6/4311836/model/5224265/document/c02722113/
The manual wrongly says that you need to connect with a USB cable to set up wifi, but in fact the 3055A can do WPS via both PIN and push button.