Asus router: "Your ISP's DHCP does not function properly"

I am experiencing the same problems that others are with Comcast and an Asus router. The same modem (Arris SB6183) and router (Asus RT-AC5300) have been used for years. Now I am getting the same "Your ISP's DHCP is not functioning properly" error. As other's have experienced, I can connect the modem directly to a computer and it will obtain a DHCP lease and IP address, and otherwise functions properly.  I have also tried changing from aggressive to continuous and normal DHCP query frequency, reset everything multiple times. Nothing works.  The same modem, with the same router, with the same router firmware version used to work for years up until about a month or so ago.

Fortunately, for the last 2 years or so Comcast has been my backup to another provider's fiber.  I have the cable modem as a secondary WAN with auto failover/failback.  That too was working up until about a month ago.  I have tried making cable primary, and everything else possible, but nothing has worked.  Fortunately I am not completely without internet, but this isn't much of a backup, and should just get cancelled as it is.  Notably the router works just fine with the fiber provider.

I did call Comcast support.  I spent an hour of my life with a technician that knew far less than I did and was useless. After all the typical questions "did I reset, etc." he had no clue.  I pointed him the these Xfinity forum pages that he had no knowledge of.  My primary purpose was to get him to send the provisioning signals as noted as a possible solution. He didn't know how to do that, and said they don't control the modem. He had no clue of the SNMP control of the modem on the provider side - that is how, among other things Comcast can remotely reset the modem.  I wasted an hour of my life on the call with the only outcome of raising my blood pressure a few points.

Someone PLEASE HELP! Does anyone from Comcast monitor these forums?  I know there are people that have technical knowledge in Comcast, I have no clue how to reach them. 

One thought is to force a factory reset on the modem and then go through the initial sign on sequence. Would that force provisioning to the modem?

Note also that Comcast recently increased the speed of my package. That has obviously been provisioned because I am receiving the claimed download speeds. The problems started before I received the email of the increase speed, but perhaps they provisioned the modem prior to sending the email?

Someone please help!