Netduma Fraser

You shouldn't need to select anything else. Not specifically that I am aware of but as long as you can get an Open NAT now through a console/on a game while connected to the R3 then you're good

Please provide a screenshot of the R3 WAN Settings page

Copy the WAN IP from the R3 WAN Settings page and enter that into the DMZ section on the ISP router

Yes please try that but also set the R1 WAN IP as static/reserved on the ISP router so it doesn't change and make the DMZ invalid. You can keep UPnP enabled

Did your ISP router/modem disconnect at the same time? Are you using PPPoE?

Are you using PPPoE/VLAN or IPv6 at all?

We don't really have any recommendations for third party equipment but any should work well

Added

On the loading screen does it give you options such as reboot, continue anyway, get logs etc? If you unplug the router from power for 10 seconds, plug in, wait 4 minutes then access can you get in then? What firmware version are you on currently?

Yes you're on the latest firmware currently. Steady Ping is a DumaOS 4 feature and DumaOS 4 won't be coming to the XR range unfortunately.

Added - it has been removed currently

Added

You said you had the R1 in the DMZ on the ISP router - this is the WAN IP from the System Information page? And you mentioned you had port forwarding rules as well, is that still the case? If so remove those and check again

Added

Yes on the router

There aren't any specific settings other than the ones that have been mentioned that would block it, try disable Adblocker though and see if that helps

Added - it has been removed currently

Always On or Auto in most cases

You don't need to use three routers, just the Linksys or the XR1000 to handle PPPoE, then put the R3 WAN IP in the DMZ of that device, reserve the WAN IP of the R3 on that device. Reboot the main router, wait 2 minutes, reboot the R3, wait 4 minutes then check again.

I doubt it, having the router specifically determine the connection method behind APs is difficult as is as they appear to be connected via ethernet. They will still work even showing offline as far as I am aware

I'd suggest following this guide https://support.netduma.com/en/support/solutions/articles/16000074717-how-to-test-your-internet-ping while downloading/streaming etc and start with a high value for Congestion Control (set to Always), say 95% then check, decrease by 10%, check, decrease by 10% etc, until you get to a value that is pretty good and then try 5% either side of that value to see if it can be improved. It's important to note that Download & Upload on Congestion Control don't have to be the same value and that you may have a better experience with differing values. This will be a more real world test.

Factory reset then don't change any settings, does it work then?

Disable QoS fully, does it continue to happen then?

Yes that's what I believe is the case, when connected to an AP, for all intents and purposes the R3 sees the device as connected to the R3 itself. As the R3 doesn't have 6GHz and thus has no way to recognize that kind of connection it doesn't display it, though of course technically the Eero gets the internet and passes it on to the laptop so it will have internet through the R3 it will just appear as an offline device.

What do you mean by the Geo-Filter method? Does it work with the Geo-Filter disabled?