During my most recent hospitalization, besides making a video about the IV pump, I was messing around with a Raspberry Pi 4G LTE router project I could stash in my hospital go-bag (I am hospitalized about 2.5x per year, on average).
I spent the greater part of this afternoon trying to get my AirPort Express to connect to the Internet and share an IP address using a Motorola SB5101 Cable modem (with Charter Internet)... and since the solution was so simple and annoyingly stupid, I thought I'd post it here, for my reference and for anyone else spending an afternoon thinking his AirPort Express is dead.
As it turns out, the cable modem (this one, and likely many others) will only remember the MAC address of the first device it recognized when you last power cycled the modem.
When the Internet went down at my condo yesterday, I turned off my cable modem, plugged my Mac straight into it, turned the modem on, and use the internet via this direct connection for a while. When I plugged the AirPort Express back into the SB5101, I just got a flashing yellow (amber) light, and in the Airport Utility, a notice that the 'Internet Connection wasn't working'.