Blue Iris

Yes, I thought it would be different subnets, and again two DHCP servers complicates the issue. What I do is find out the range of DHCP addresses available in the router settings, then avoid that entire range by giving the cameras a static IP away from that range. Why static ? Because the router might give a different automatic DHCP IP address to each camera from that range when it or the camera is rebooted. That means BI5 can't find your cameras.

So if the DHCP range/window is found to be 192.168.0.10 to 192.168.0.50 then I could start my first static camera IP at 192.168.0.100 and increment the last number by one for every extra camera. BI5 can then be set up to find your first camera at 192.168.0.100, the second at 192.168.0.101 etc.

Is there a way to tell my camera to default to the att router subnet first?

I would say not. My cameras tended to come with a single default static IP address, which was different for each manufacturer. For example ALL of my Dahua PTZ's would arrive on 192.168.1.108 which was a faff for me as I use 192.168.2.x for my home LAN. I would then have to put a PC on the same subnet and log into the camera with a browser to change the static IP for the first camera to 10.0.0.2 to match my cctv subnet. It would then disappear from the 192.168.1.x network. I could then plug it in to the cctv network with the NIC at 10.0.0.1 and BI5 would find it at 10.0.0.2

It is logical when you understand the rules, and they are fairly basic.

What I presently have is two Asus routers in "AImesh" network mode at opposite ends of the house. One of them replaced my ISP's cable modem/router. Either Asus router can hand out automatic DHCP IP addresses from the same DHCP range, and they work together to manage this. The BI5 PC is plugged in to one of these routers. The BI5 PC then has a second NIC at static IP 10.0.0.1 and that plugs directly into a POE ethernet switch. There is no router on the cctv network, and the cctv network cannot contact the internet. That prevents your cameras calling the Mothership. BI5 is the bridge between the networks, and they are otherwise physically separate.

Please note that I am not a Network expert. I've simply tried everything until it worked, or as I like to say "some experimentation may be required.".

If your Deco is issuing IP addresses then the simple thing to do is stop it, this way your main router issues all the IP addresses or block off part of the range so that the main router issues a range and the Deco issues the other part of the range?

I'd personally just stop DHCP on the Deco, this was you can see all devices connected when checking your main router.