I did some further testing.

On my Archer C6 V2.0 (EU) router, when I set the 5 GHz bandwidth to 40 MHz, connection is stable but I am only able to get ~160 mbps.

When I change it back to 80 MHz, I am able to attain ~250 mbps but connection is very unstable.

Here's a list of drivers I tested, all of them have the same issue:

• 1030.38.712.2019 (Windows 10 default driver)
• Archer T4U(USEU)_V3_181023_Win
• Archer T4U V3_TP-Link driver_1030.22.0202.2018
• 00011389-RTLWlanU_WindowsDriver_1030.29.1102.2017_DUA_tplink_0719

Weird thing is, the problem does not occur when connected to my smartphone's 5 GHz hotspot and my previous router (Netis DL4480V1).

@MushyPotato 

How to update 1.3.1 ?

I have the same problem since firmware 1.3.0 input. Beta 1.3.1 has fixed the problem, but now it is 1.3.4, if you have beta, don't install it !! I don't know if it's the router's fault or the T4U cards. Smartphone and TV works well at 5ghz. What is wrong, router or card ?? 

when the card and router are close to each other, it also works well, but it makes no sense, because then I would use a cable.

@Kreco I updated to 1.3.4 and I have no issues whatsoever.

@kiNP You can get 1.3.4 for C6 V2 EU instead https://static.tp-link.com/2020/202006/20200605/Archer%20C6(EU)_V2_200512.zip

@MushyPotato i'm using1.3.4 and have the same issue 

@kiNP 

I restored 1.2.1 and I have no problems, I gave it up, because I assumed that they would not do anything about it, but apparently beta 1.3.6 is already circulating. I don't want to test it, because then it's hard to restore older ones. (via TFTP) and the subject disgusted me

@Kreco set auto in channel width and channel