I've never seen network components act so oddly.
My guess is that the cable modem is mostly dead and any more than a few random connections makes it completely give up.
So it's not necessarily sharing the network, but the result of so many devices reaching out to the Internet at once.
This would explain why the routers sometimes seem to work for a bit, too. Until nothing does a few minutes later.
Cable company didn't even ask us to power cycle the modem. They just shipped us a new one. 🤯