So I was doing some new cable this weekend to put in a wired connection to the cable modem. In the process, I unplugged the coax cable going to the splitter that feed the main Gateway box as well as the cable modem, and inserted a keystone jack and short patch cable. After everything was done, I could watch TV on the TV attached to the Gateway box, but the cable modem would only go as far as the flashing the second of the house symbols (house with down-arrow?).
So now none of my other TV's were working. Went to the "Local Network" in the main Gateway box, and it says the DNS is not available. Talked to a support person last night, and he suggested (after trying a number of things) I could remove the splitter and connect the Gateway box directly to the wall plate (i.e. remove the cable modem from the mix), and I should be able to watch TV on all the TV's in the house, at least. That worked, so at least it's something. But why would the main Gateway box be able to get a signal to show TV, but not be able to get a DNS address or anything else?
I'll post in the Internet side about the cable modem, I guess... I've got a technician coming over tomorrow evening, but if I can avoid that, I'd rather solve it on my own.