Problem with 5268AC Modem (fiber) and Asus AC-88U / AX-88U routers

Hi All - I'm supremely frustrated right now as I don't really understand what's wrong.

So the past year or so, I've had the ATT 5268 fiber modem connected to my Asus RT-AC88U router.  I set it up day 1 after the initial ATT install.  Everything's been fine since then.  It handles all the routing to the multitude of devices in my home.  Then yesterday morning, the internet went out.  More specifically, the internet (wired / wifi) went out from the router.  The 5268 modem's wifi works and from what I can tell, my laptop could access the internet and run a speedtest while wired directly to the modem.

Aside from a recent firmware update to the router (a few days ago), no config changes had been done on the modem.  After sitting on the phone with tech support as they rebooted / re-loaded firmware to the modem, there was no luck.  The router interface would show a DHCP IP and the public WAN IP, but no internet connection (wired or wireless).  At that point, I thought there was something wrong w/ the router so I did a factory reset and it was behaving like it was bricked.  I said ok - I will deal w/ this router later, and I ordered a new Asus RT-AX88U router (I use Asus DDNS, so I prefer to stick w/ the brand).

It came in today and I have done the following based on various similar solutions I've seen.

1 - I setup the router w/ the internal LAN IP I prefer (192.168.0.1) and configured the basic wifi stuff, updated the firmware.

2 - on the 5268, I tried a few things - but ultimately the last thing I did was a factory reset.

3 - I turned off the wifi on the 5268.

4 - In the Firewall settings, I selected the new router (via it's MAC address in its name) and selected the DMZ+ option.

5 - in the Advanced settings, I unchecked everything.

6 - Saved the settings and rebooted the router.

Also I'd like to note I changed the ethernet cable going from the modem to the router and also ethernet cable going into the fiber input of the modem. 

The problem I am having now is that again, the router is getting a DHCP IP address, a public WAN IP address, but the internet starts to work for a few sites and then stops working.  Like Google and Youtube will work for a bit, but if I try to go to any other site, the browser times out.  I just don't get it.  It's a brand new router.

Is it possible that something is wrong w/ the 5268 modem?  Maybe the LAN ports?  <- I'm not sure about this either, as I have another router in another room that I use as an AP and that is still working when the source ethernet cable is plugged directly into one of the 5268 LAN ports.  After going through the troubleshooting w/ tech support again, ATT said they're sending a technician over on Saturday.  Meanwhile, just wondering if anyone has any ideas.  Setup seems pretty straight forward and frankly everything was just fine for like a year until yesterday.  Not sure what's changed.

Thanks.