@robi217 is your router the EU version? Did you just use the firmware 'as is' downloaded from the TP-link website?

@spies 

Yes, I have EU version. I downloaded the firmware from the official TP-link website. The 1.2.1 is the one with the name "Archer C6(EU)_V2_200110", with release date of 2020-01-23. But I could not flash it directly (from 1.3.0), if you simply try using the web interface, it will just say invalid type. TFTP also won't work directly, even though it will flash, it will just stay at 1.3.0 like nothing happened. The only way (for now) is to flash OpenWRT via TFTP method, and when you have OpenWRT running, you can TFTP flash 1.2.1 again.

Please note, it may void your warranty, as you have to flash OpenWRT first, which is a non-official system (even though the TP-Link firmware itself is based on OpenWRT, just check the .bin file with Notepad++ or 7-zip).

@spies 

Thanks,  I am aware of all that. I just wish to have next three things: stable 2g and 5g, ability to turn off Onemesh (no hidden SSID during normal WiFi opration), and time schedule that close WiFi emits completely. Now in FW 1.30, it still emits hidden ssid even though I turn off the WIFI. 

@robi217 This hasnt worked for me, I'm on openwrt, but am now unable to return to stock, either via the TFTP method of flashing through the openwrt WEBUI, TFTP method ignores the file and if I force it though the WEBUI of openwrt, it won't boot, powerlight remains on and nothing ever happens.

@robi217 I've done it in the exact way I got openwrt on there but it just ignores, the file, really odd.