I fixed this. AFAICT it was the Ethernet cable that connects the modem to the main Deco -- somehow it went bad or was marginal. I replaced it and everything is working again.

I've had a 3-node Deco XE5300 set for about a year, and it's worked fine until yesterday. In the middle of the day my Internet went out -- i.e. the wifi network was still up, but I no longer had...