TP-Link Engineer discusses the Android/Chromecast Router Reboot/Disconnect Issue
Above is a link to a engineer's writeup as to what appears to be Google software causing mass reboots/disconnects/instability across multiple router models/brands. Maybe this will help some on here that seem to be suffering from the same issue.
"Following initial research and investigation, our Engineering Team is confident that they've determined one of the key origins of the issue. From what we have gathered so far, the issue appears to be related to some recent releases of Android OS and Google Apps. This issue stems from these devices' "Cast" feature, which sends MDNS multicast discovery packets in order to keep a live connection with Google products such as Google Home. These packets normally sent in a 20-second interval. However, we have discovered that the devices will sometimes broadcast a large amount of these packets at a very high speed in a short amount of time. This occurs when the device is awakened from the "sleep mode", and could exceed more than 100,000 packets in a short amount of time. The longer your device is in "sleep", the larger this packet burst will be. This issue may eventually cause some of router’s primary features to shut down – including wireless connectivity.
Generally, you will need to perform a reboot to release the memory to solve this issue. You may also try disabling the “Cast” feature on your Android device to help mitigate the issue until an update is released to permanently fix this issue on the device itself."
Current mitigation seems to be accessing your Chromecast and turning "guest" features off or disabling anything that can act as a "Chromecast" on your network.