RAX40 Stops bridging internet to LAN within seconds

My RAX40 has stopped bridging internet to LAN within seconds of each reboot, reset, and/or release and renew for the past couple days. I'm hoping someone here might have ideas for me.

Router was purchased in Nov 2019.  It currently has firmware version: V1.0.3.80_1.0.1  (since yesterday -  updated from Firmware Version 1.0.1.36 - I think).

The problem exists on all devices connected to it (Windows & Linux PCs, XBox, Android phones, iPhone, IPad, Smart TVs, Google Home speakers).  All devices (connected by 2.4GHz or 5GHz WiFi, or by Ethernet) have no problem connecting to the router. I've had no problem accessing the router admin, and the devices seem to have no problem connecting to each other. Most things on WiFi show "Connected, but no Internet"; the XBox shows "DNS isn't resolving Xbox server names". PC web browser responds "Cannot reach server" for anything outside my home LAN.
The router also appears to have no problem connecting to the internet through the Spectrum Cablemodem (model E31T2V1). The router receives an IP from the ISP, by DHCP. The "Show Statistics" page (Advanced Home>Internet Port>Show Statistics) of the router admin shows some increment to the Tx and Rx for the WAN Port, suggesting communication between them is still happening.

I have used the DNS servers provided by the ISP by DHCP, and I've used Googles public DNS (8.8.8.8 and 8.8.4.4), with no difference in success. Lengthy discussions with ISP tech support have confirmed they see solid connections with my modem, and the modem is reporting to them solid connections with the router. After reboots of sometimes one, sometimes both devices, I can usually get ONE or TWO web pages to load on my PC (google.com, Youtube.com, IMDB.com have been my go to test loads) in the first few seconds after the modem and router synch up, but I usually can't get anything past that.

At the suggestion of the ISP tech yesterday morning, I connected PC directly to cablemodem, bypassing the router, and quickly got a solid connection that lasted for over 30 min, before I reconnected the router to troubleshoot that (not much use to the rest of the family, if we can only connect one device at a time, through ethernet).

Following that, I tried a factory reset on the RAX40, and updated the firmware from the initial load from last year, to the current version: V1.0.3.80_1.0.1 .  After redoing all of my settings (Wireless SIDs, passcodes, DHCP IP ranges - nothing too outrageous), everything seemed to work perfectly fine, all day long, and halfway through the night. About 20 hours after the updates, everything started again as described. I thought I'd check in here, to see if anyone had any ideas.

Side note: This all started just over a week after getting a replacement Cablemodem from spectrum (same model as last one). The previous issue was that the cablemodem kept losing synch with the ISP. Their first fix was replacing the bad (according to their tech) cable between the street and the house, then when it went out again later in the day, they swapped out the modem.

Thanks in advance for any input you may have. Let me know if I left out any pertinent details.

~Josh