Ok, so I checked it again, and made sure to note the status LED, and put the serial debugging messages back in.
I started with the antenna connected. The electron flashes dark blue once, flashes green, flashes cyan, then connects and breathes cyan.
I then disconnect the antenna. After a few seconds, the connection fails and the led goes to breathing white after Cellular.off() is called.
The antenna is reconnected, and when the reconnect timer expires, the program calls the cellConnect() function. The led breathes white for a second or two, then flashes green. It restarts this white, green cycle about every minute or so. The connection times out as expected, and the electron returns to it's breathing white, connection failed state.
I hit the restart button on the electron (antenna connected) and the electron connects and breathes cyan, so I know things are still working in general.
Is the modem some sort of "discrete" thing that I could reset or power cycle manually?