@DaveULS - I am also having this problem. Every day or two internet drops off. I have the axe5300's. I had QOS enabled  I am going to turn that off and see if that helps.

  @DaveULS 

my XE75 system keeps disconnecting from the internet. The art modem is fine. This is a fairy recent problem and mostly affects IOS devices. Sometimes my IOS devices lose internet, the Deco app reports not connected to the internet but my fire tv sticks still keep playing content. This happens 8-15 times a day now. 

  At least one or two of my ex75 units still go offline every few days. Sometimes it even days I have no internet when I do.   What is causing this and for do I fix?? 

  @DaveULS -
My issue might be provider related. Spectrum seems to have stability issues on a daily basis. I need to monitor it more closely I guess.

Here are some ideas you can try:

1. The next time you notice it go down note the date and time. Log into the web version of router settings and go to Advanced -> System -> System Log and grab all of the CRITICAL AND ERROR logs. Upload to this forum. Maybe someone with an understanding of them can help deicpher the issue.  Also grab and make and model version of the units. 
2. Set up a daily reboot schedule on your system. You may find this helps with stability. 
3. Try disconnecting one of the misbehaving units and monitor for stability.  If the issue happens again disconnect another one and monitor.  Perhaps a faulty unit is causing some issues. 
4. Simplify your setup, maybe turn off Guest Network, IoT network, QOS, etc. Then monitor for stability.
5. When you have stability issues again check your interenet provider on DownDetector.com.  If your provider is having problems consider calling them and report the issues if it is causing you ongoing headaches. Be prepared for them to what to come out and inspect things. If you want that or not. 99% of the time it's simply on their end. 
6. All of my units are using Ethernet Wired backhaul. Not sure if that makes a difference or not. 
7. Try running the network optimization in the phone app. Perhaps your WiFi channels are experiencing massive interference with neighbors.

I had a Netgear router that I ran Tomato on for 10 years and never had any stability issues. It was rock solid. I may end up dumping this system at some point and go to something more robust like Ubiquiti, but I am not at that point yet.

  @ArthritisEye Exact same situation for me, I had Tomato for years and no problems, had dd-wrt at another home for over a decade with zero problems.

But with this device, the lack of forum communication from the devs isn't confidence building. Heck just let us know they're alive. 

As for advice, I agree set up a reboot schedule every night and simplifying the network setup is key; if it's not vital, remove it. Also, checking your channels to ensure there isn't interference, since that's usually a cause. And, don't trust your internet provider, most will gleefully blame your equipment than take personal responsibility lol. Last, try making sure you research and buy your own modem, as the ones the rent you are garbage 99% of the time.

  @DaveULS - Just had another random drop on my internet today. Does anyone know if TP-Link is looking into if there is a potential issue?  I would be happy to provide router logs to the engineers. 

  @DaveULS I have a support case with open with them.  The other day they sent me a different firmware to try.  I did have one XDE75 go offline about an hour after installing the firmwre, but they havent gone offline since, except for one additional time when my XFINITY internet went down, which made them go RED.  As soon as XFINITY came back, the XE75's were back online.  I provided all of this information to my support case and havent heard back yet....

  @DaveULS Heard back yet?