When I plugged the SD card into my laptop, I saw that it contained a tiny boot.src file and an empty @ directory. This didn’t look right. So I followed these instructions to manually configure the SD card: Turris Documentation
Now when I plug it in, the status light turns on for a few minutes, and then begins to double blink (blink twice) over and over forever.
EDIT: After another attempt at manually configuring the SD card, I plugged it in and waited hours for it to eventually become available on the network. I finally reached a first-time setup screen! I selected Router config mode, and then it went offline again. After more hours, it was not coming back online, so I disconnected it from my Turris Omnia router and connected it directly to my modem. I had to reboot the modem and wait a long time to be able to connect, but eventually, connection! I finished the configuration, but during updates…it goes offline again. This time, I know what to do. After some waiting I am sending this message through my freshly configured Turris MOX router. PHEW.
Edit 2:… Now I only have to deal with multiple daily crashes and having to physically reset the router by hand: Reboot doesn't work - #25 by kxra - SW help - Turris forum