im using OpenWrt 19.07.6 r11278-8055e38794 on my TP-Link Archer C6 v2 (kernel version 4.14.215).
From beginning, the router crashes wifis (both 2,4 and 5Ghz) at the same time for just a minute... this is happening usualy during the weekend or at night where in around appartments lots of people are at home and using wifis for lots of devices... i usualy swithc to another channels not to be jumming, this usualy helps for week or two, then it all starts all over again...
while the WIfi is down, LAN works just fine on the router (has connected TV like this which does not drop connection).
Always, at the same tie wifi is don on both 2,4 and 5ghz, it disconnects all devices (laptops, smartphones, etc) - so i know its not enddevices but the router.
System menu in LuCI has the flash page, where you can upload the sysupgrade image to the router, (once you have downloaded the image from the OpenWrt download site to your PC first.)
You might try 19.07.8, which would be the newest on your current 19.07 series. It might already help a bit.
Or go directly to 21.02.1