@LukasTL 

Try the beta FW from this thread. Disable OneMesh feature and test.

You can always revert back to the current FW if this doesn't help.

@terziyski Thanks for the answer. In the thread, what you sent there is firmware 1.3.0 Build EU 20200402 rel.52000. But now I have a higher version of firmware - 1.3.6 Build 20200902 rel.65591. So should I try a version of firmware, that is older than the newest? 

@LukasTL 

In the Kevin's post (Recommended solution) there are 3 options - choose the right one according to your region:

@terziyski Thanks, already tried with beta firmware recommending in the thread, but still the same as before. OneMash function disabled too.

@LukasTL 

Make sure your wireles security settings for both bands are:

Check for wireless interference arround you, installing "Wi-Fi Analyzer" app on your phone and scan.

If there is, change the wireless channel to less crowded one (both bands).

Also it's a good idea to check your ISP connection - if you can plug your computer direcly (bypassing C6) and test if the connection is stable.

@terziyski I checked WiFi interference and set the best channels for me. Wireless settings I have the same as you sent in the screenshot. And if I unplug the cable from the router and put it directly to my computer, the internet connection is stable. But If I test the internet connection via router Archer C6, but not in one moment with another device, the internet connection is stable too. Problem is when I test the internet connection in ONE moment with two devices (one device is connected via cable and the second via wireless connection). Then the internet connection is not stable (on one device - it doesn´t matter, on what device. In one time on device witch is connected via cable .. in a second time on device witch is connected via wireless. So this is a problem for me. When two devices are communicate with the router in one moment.

@LukasTL 

I just realized the way you try to test your internet speed. I have mimssed that from your firts post.

You should not test your internet speed from two different devices at the same time.

I am quotting a former employee of speedtest.net:

"

Q: Could running multiple speedtest.net tests at the same time slow down your results?

A: Not only will it lead to slower results, but those results will also be incorrect. Speedtest.net has to compete for bandwidth with everything else that could be using your network connection. So if you have a YouTube video downloading at, say, 1mbps while you're running a speed test, then your Speedtest.net result will be about 1mbps slower than the maximum speed your connection is delivering.

@terziyski Ah okay I understand. But when I had an old router D-link DIR600 there was all OK. For example, I have an online conference with colleagues via WiFi on a notebook without any network problems. During the conference, I started PC via cable and I browse the internet in the one moment with conference and then conference on the notebook via WiFi started freeze and unstable. But with old router Dlink I didn´t have this problem. 

@LukasTL 

On the next conference call try to connect both devices (PC, notebook) via ethernet cable to the C6 so you can isolate the issue and see if it's related to the C6 Wi-Fi or the C6 itself.

Or if it's possible make the conference call from the PC and browse the internet from the notebook then observe.

If you keep your old router replace them and test to confirm if any difference.