RT-AC66U WiFi unstable

[sirwifi]

My RT-AC66U 2.4Ghz WiFi dies every couple of days. It's currently running Merlin 378.51 but that has been the case for a while. I never used the stock Asus firmware, I just flashed to Merlin from day 1, so I'm not sure how well that works.

My setup is very simple: AP Mode behind another router, a couple of wired clients and various WiFi clients all on the 2.4Ghz radio (Windows 8.1 laptop, Chromebooks, Android phones and tablets).
I use a static channel setup, not auto. Control Channel 6, Ext Channel Above (so 6+8; I ran an Wifi Analyzer and that area is free around my house), 40Mhz, N only mode, WPA2/AES, short preamble. I don't think I made any other changes.

The symptom is that WiFi just stops working, some clients seem to still be connected but they can't pass any data, while other clients just disconnect and they can't reconnect until I power cycle the AP. Wired clients still work, it's just the WiFi part that stops working.

Has anyone else seen anything similar? Any suggestions on how to troubleshoot it when it gets in this state? I have ssh enabled and I can access the Web UI, so if there are any logs I need to look at, or processes, I at least have access from the wired clients.

 

[L&LD]

Install the latest RMerlin 378.52 firmware.

Reset to factory defaults. Configure the router manually and minimally to secure it and connect to your ISP.

Use new ssid's for both bands.

Test for best throughput (how fast and how stable and consistent) on each channel manually. Don't rely on programs like inssider. Simply change to each channel, reboot the router and copy a large file from a wired computer with a fast hdd or ssd to a wireless computer with the High Performance option selected in the power plan and preferably connected to the AC wall power too from a stationary location (for testing).

For the 2.4GHz band, use the 1, 6 and 11 channels only for your testing in both 20MHz widths and 40MHz widths if your WiFi environment is not crowded by other routers. For the 5GHz band, test every channel available and connect as many devices there as you can. If you have no 5GHz capable clients, turn off the radio in the gui.

It is also worth turning off the modem and all other networking devices for at least 10 minutes and preferably 30 minutes or more. Turn on the modem and wait for 5 minutes for it to fully boot and connect to your ISP. Turn on your main router and wait another 5 minutes. Now, turn on your AP and after 5 minutes, turn on each device you have. Do not put the devices to 'sleep'. Physically power them off.