This is the 2nd Archer C6 (US) that I've gotten from walmart.com.  They replaced the first one because of all the connection problems.  This new (replacement) unit seems to be having similar problems.

I'm using both direct/wired connections (7-8 clients), and 10-14 WiFi connections (almost exclusively 2.4gHz).  I'm mostly having degraded connection issues with WiFi.  It'll usually happen with 1-4 devices initially.  I see "buffering" and slow connections with streaming services.  Mobile browsing will become extremely slow to display a page.  Sometimes the entire connection to the internet will be lost.  I never see any errors on the router logs, and the WiFi connections rarely completely disconnect, they usually get very s-l-o-w (but the connections do sometimes show "no internet").  It seems to be mostly WiFi connections, but sometimes wired connections will have similar issues.  I don't think it's my ISP (broadband cable) because some connections continue to operate normally.  All the lights on the router remain green.  Since I have a smart-house with Amazon devices (echo shows, echo pucks, Samsung SmartThings, etc.) -- I also  have a lot of IOT WiFi devices (outlets, thermostat, garage opener controller, video door bell, etc).  When the problem occurs, it almost never impacts all of these devices at the same time, but usually 3-6 devices will have connection issues.  It has impacted Roku devices (mostly WiFi ones, but I've had the issue with wired Roku as well).  My house is not that big, and the signal strength from the router (when working) is always quite good.

I have tried the original firmware.

I have upgraded the firmware to the latest version

I have tried increasing my DHCP lease to the maximum time allowed

I'm using WPA2-PSK security

Encryption is AES

I have tried limiting channel width to 20MHz

I have tried switching to Channel 11

My transmit power is HIGH

I am currently running only 802.11n (but also tried b/g/n mixed)

I rarely connect using 5gHz

I do have a 2nd router that I use an a backup/alternate access point.  When the WiFi problem occurs, it seems like that router's WiFi works as expected (but just the act of disconnecting and starting a new connection may be a factor).

Every time that I experience any of these connection problems on any of my client devices, I eventually need to reboot the router to solve the problem.  Rebooting the router always fixes the problem.  I never need to reboot the cable MODEM, nor am I seeing an errors being recorded with the cable MODEM.

I see other similar connection problems posted on this site with the Archer C6.  I strongly suspect you have a design or firmware issue.  If you can't help me find a solution, then this router will need to be returned.  I can't spend hours every week trying to debug and solve the problem.