No Internet - DHCP / IP Configuration Issues?

We were away from home for a weekend and on returning, found that our wifi network was no longer connected to the internet.  Some info on my hardware setup:

Arris Surfboard SB6190 modem (self-owned, compatible with Xfinity service)

Linsys Velop mesh wifi system, connected via the single ethernet port on the SB6190

Connection status lights on the modem are all lit as expected (power, upstream, downstream, online)

There have been no changes or major issues with this configuration for 3+ years of use.  After the usual steps of restarting the modem, restarting the hardwired router and wireless nodes, disconnecting the modem from coax and restarting, etc. I am still not able to connect to the internet. 

As a next troubleshooting step, I have disconnected the router and connected a computer directly to the modem via ethernet cable.  I am not able to reach the internet from this computer.  When running the windows network troubleshooter, I receive the error "Ethernet does not have a valid IP configuration."  There is no manual/custom configuration on this network - it is using standard DHCP to configure an IP.  

I have attempted to connect directly to the modem's management interface but get no response in the browser from the management IP (192.168.100.1) so I am not able to review the modem log, or perform any configuration/hardware reset.

After speaking with Xfinity support over the phone, they were only able to issue a restart signal to the modem and confirm that it is properly connected.  They then proceeded to make claims about a "worn out boot file" because I have "too many devices on my network."  They suggested that I contact the manufacturer of my modem to resolve this issue -- which is clearly false, as Motorola/Arris has no access to the Xfinity network and cannot access my specific hardware or configuration.  Xfinity also has no access to my wifi setup (since it is self-managed, not Comcast hardware) and any claims made after that point are irrelevant to the issue at hand.  The same issue -- cannot connect to the internet, cannot issue a valid IP configuration for my client machine -- is occurring on a single computer connected via ethernet to the modem.

I understand there may be a need to re-provision the modem by Xfinity (not Motorola, as incorrectly claimed) and need a competent support tech to investigate and provide a reasonable corrective action.