I've actually been talking to TP-Link Support and they had the same questions.

ISP: Spectrum

Cable Modem is Arris Surfboard SB6183

When the Main Deco has a red light, it is unresponsive to IP calls to dashboard, as it is fully crashed. So I can't get any settings off of it. The issue is not the internet however. As I've never had issues from the Modem.

Possible Solution: My Main Deco ran Ethernet to a switch, which ran ethernet to a number of local devices. One ethernet ran 150ft to a linksys wifi router in a backhouse with it's own wifi SSIDs for a separate wifi network. This looks to be freaking out the Main Deco. Once we removed the remote linksys wifi router we have been online ever since. We therefore moved the satellite w7200 deco to the backhouse and connected that to the ethernet cable instead. It puts the backhouse on the same SSIDs, but was the only fix we could consider. So far the Main Deco hasn't crashed.

There are some intermittent issues with our Roku devices connected to the Main Deco wifi, where it thinks internet has gone out very quick, but nothing like a full crash.

  @David-TP 

Yes, you're schematic is on point.

When it crashes, it is solid Red.

2. When I try to check at time of going red, I usually use my PC that is networked in through the switch, but the connection becomes impossible. I hadn't considered using the app.

3. Yes, every time a solid red light showed I could not access the dashboard.

So the next step for testing, reconnect the linksys router, when the Deco crashes, try to see what Deco APP>More>Advanced  shows?

  @David-TP 

Alright, I will try that.

I'm still not sure how we factor the Arris modem in, since the issue was resolved (so far at least) by removing a Linksys router from the switch.

Also, on the hard crash it seems to lose the log files, and it temporarily resorts to thinking the year is 1969. Here is one such log file I was able to recover after a restart from crash (unplug/plug). I would have uploaded it but the log or txt file failed to upload. Also, when it crashes and the red light appers, I cannot connect to it directly by ethernet from computer, nor can I connect by Wifi (so maybe the app won't work?), as all our smart home features go offline and our alarm system has to resort to cellular backup (typically how we first discover the Main Deco crashed again)

I tried uploading the log file but it gets rejected. It is a text file. I tried changing back to log file and it rejected that as upload too. (nevermind, came back and edited by converting to a pdf file)

Hey guys, I never got a ticket email, and I've been searching for it. In short, my conclusion is that I got a Lemon deco and was hoping for a replacement shipped out. There is no way this thing can be this bad without being broken. I have had one minor improvement. Now when the red light comes on and thinks there is no internet -- of course, there is -- I can actually still connect by IP to the Deco, so it hasn't crashed yet. I can then reboot by IP, and it starts back up.... only to crash within a minute or two of getting the green light. Is there any beta firmware updates, as I've never had wifi hardware as bad as this. It has to be a lemon, that's my only remaining theory. Can I get a replacement? I'd send logs, but after every crash they claim to be from 1969.

Also, according to my Home Security, it had to use cellular backup once (1 time) in 2021 due to a power outtage. With the Deco, it has used celular backup 168 times. That's how little this thing can remain online.