FVS-318N; FVS-318Gv2 and Arris

Very interesting that a router in between is getting these units working correctly.

And it's not the cable modem, but the configuration of the head-end equipment by the isp, typically made by Arris.

Does anyone have a Frys nearby?  This $20 managed switch might be able to act as the filter and allow the 318G to work normally:

https://slickdeals.net/f/9811851-netgear-prosafe-8-port-gigabit-web-managed-plus-switch-gs108e-15ar-frys-starts-2-26-6-hdmi-cable-0-95-arlo-1-hd-cam-system-120-klipsch-r14m-speakers-97-pr?src=SiteSearchV2_SearchBarV2Algo1

Or order it from newegg:

https://slickdeals.net/f/9779091-netgear-prosafe-8-port-gigabit-web-managed-plus-switch-gs108e-22ar-ac-newegg?src=SiteSearchV2_SearchBarV2Algo1

These managed switches should be able to 'filter' the packets even if they aren't using any other managed switch options.  Cheap fix to the problem.