R7400 disconnects from Internet on every WAN DHCP lease renewal

Thank you for your reply.

The cable between the modem and the router is CAT 5, but it's only 6ft long.  The coax I can see (because it's not in the crawl space) leading to the modem all appears good.  The signal quality seems to be fine, whereas the DHCP-renewal-disconnect problem seems to be protocol/logic related.

I've previously factory-reset both the modem and the router and reset the network more than once, but I factory-reset the modem again today, then reset the network in the specified sequence.  The Internet disconnection on DHCP renewal problem was present after the network reset with the router in its factory configuration and is still present after I reconfigured the router to my settings.