Thanks @gr5. I flashed the recovery.img and booted from it. After a while, the dial went green and the LCD said to power down and remove the SD card. After doing that, rebooting normally, and going through the initial setup, the printer was running firmware 4.3.3.20180529. I was then able to upgrade to 4.3.96.20190313, then 5.2.8.20190320. I haven't run a print job yet, but the menus and network connection seem to be working.