As this is the only slightly relevant post about this I see online, I wanted to provide an update: the three pins by the ethernet port are in fact serial.
The one closest to the reset button is RX, then TX then ground.
115200 8N1 no flow control worked for me.
I have one of the ones from comcast, which responds to pings and nothing else.
It has a uboot password and doesn’t drop you into a shell so I haven’t been able to talk to it yet.
Does print logs from init, it starts mosquitto and sshd. Pity I can’t talk to them.