@spies I'm sorry, but I don't know if you are talking about OpenWRT or stock firm. What beta have you received?

@spies I'm almost sure you can revert from OpenWRT to any other old stock version, but I have not tried. Be careful.

@adios 

I just tried TFTP flashing from TP-Link firmware 1.3.0 to OpenWRT 19.07 and then TFTP flashing from OpenWRT 19.07 to TP-Link firmware 1.2.1, and I can confirm that it works. Now I'm on 1.2.1 again, and my router works just as well as before updating to 1.3.0.

If you want instructions for TFTP flashing, just refer to the OpenWRT page:

https://openwrt.org/toh/tp-link/tp-link_archer_c6_v2

I may also add that for some reason my laptop's built-in Realtek LAN did not work with the router, the flashing process was stuck on 72%. So I just used my desktop PC, which has an Intel card, and it managed to finish the flashing process without any problems. Also don't forget to force 100MBps connection and connect a switch or router between your PC and Archer C6, because otherwise it just did not start flashing.

@Will 

Thanks for the info, it's nice to hear that, but I think I will just stay on version 1.2.1. I don't need OneMesh anyway, and now the router works fine for me, both on the 2.4GHz and the 5GHz band. I also did not experience the DHCP problem that 1.3.0 aimed to fix, so I really don't have any reason to update the FW at the moment :)

Do you think that US version of firmware also emits hidden ssid all the time? 

@Fane 

I think that a secondary WiFi network is needed for OneMesh functionality, so if your router already has a FW with OneMesh feature, it is very likely. Anyway, you can use WiFi Analyzer on an Android device to find networks with hidden SSID, if you have one on the same channel as your normal network with a very similar signal strength, you can be almost 100% sure that it is running a secondary network with hidden SSID. Personally, I don't really care about security problems with that (I live in a rural area, I don't think anyone will ever even try to get into my network), but performance is a much bigger problem for me. Because if you have a router with only one radio (actually one for 2.4GHz and one for 5GHz, in the case of Archer C6), you can only transmit WiFi on only one channel. Now, if your router also transmits a secondary network with pretty much no functionality (except if you actually use OneMesh), it will mean that it also has to be on the same channel as your primary network, so the router will actually make interference for itself. Of course, if you don't use OneMesh, the secondary network will barely have any traffic going on, so it is not a really big problem, but still, it just doesn't make sense to have this feature turned on if you don't need it.

Anyway, there are a few differences between the US and EU version, because the US version has a 16MB flash, while the EU only has 8MB of it. So you can't just swap FW between US and EU versions.