I'm in the process of gathering the information you've asked for, but I wanted to go ahead and answer as much as I could now.
Hardware versions, firmware versions, etc., coming in another post. I had gethered all of the info, and started creating a post with it, then lost the post due to battery running out on laptop and issues with my power supply on my MacBook Pro.
I'm in USA, North Texas, DFW metroplex.
I have one satellite deployed. Router is upstairs at back of house, satellite is downstairs at front of house. Most cameras are downstairs, but they are mounted high. So they are almost between floors. The longest distance from any camera to the nearest of the router or satellite is about 30 feet.
I cannot control what the cameras connect to - so I can't test with them connected to router only. There is no way to disable them from connecting to the satellite.
There are approximately 30 devices connecting via WiFi, to both the router and the satellite. About 20 of them connect to the router, 10 to the satellite. Most of the cameras are connecting to the satellite. I think only 2 are connecting to the router.
I was using DHCP with MAC address reservations, but after going back to static IPs on all cameras they are more responsive. I have not seen an improvement in viewing the DNR-312L UI though - they still randomly connect/disconnect, with little to no change. When I say the responsiveness has improved, I'm referring mainly to the web UI. There used to be very long delays loading the first page, and all subsequent pages. Now most pages load much faster, but not as fast as it used to be with my previous router (ASUS RT-AC5300).
You mention testing cameras with UPnP and UPnP Port Forwarding both enabled on ALL cameras. I have UPnP enabled on all cameras, but I do not have UPnP Port Forwarding enabled on any of them, as that would require me to configure the ports. As soon as I check that checkbox, I get three new fields for the port numbers to use for each type of connection. I would need more information on how I should set those - do I simply make up unique port numbers for each type of connection for each camera? I did look at the pages you linked to, but did not find info on how I should set these ports.
Distance between the router and satellite is over 30 feet. Probably closer to 40 feet. As for channels, 2.4GHz is using channel 11, 5GHz is using 36. These are the least interference channels based on other wifi in the neighborhood. There are probably 20 neighboring wifi networks, but none are very strong in comparison to mine. For security I am using WPA2 and AES.
I have tried disabling MIMO and Fast Roaming with no change in behavior. I don't see how Daisy Chain would be related at all - that setting is used with Ethernet backhaul, which I'm not using. It controls whether you can daisy chain from one satellite to another for the backhaul connection. I am at work right now, I don't know how I've set IPv6 or 20/40Mhz Coexistance.
One thing I have noticed that I thought was odd - all of my cameras are adding port forwarding rules via UPnP over and over again. Depending on how long its been since I've restarted the router, there might be 10, 20, or 50+ rules, with multiple rules defined for the same camera for every camera. The Orbi doesn't allow clearing these out, I'm pretty sure that the only way to do that is to restart the router.