Please note that there are different version of the R6220. You won't be able to identify that from the outside. But after booting OpenWrt you will see error messages for incompatible versions. I posted in an old thread. But due to the ordering of threads in this forum nobody reads old threads anymore.
You can find more details over here:
I have three R6220 - two of them work fine and one cannot configure 2.4G WIFI correctly as the MAC address cannot be found by OpenWrt. Somebody created images for three different versions of R6220 but there is no description of what is contained in those files.
The main image which is released in 18.06.1 (as of today) only supports one of the three known versions. Thus as long as nobody investigates more on this topic it is just trial and error.
Going back to stock firmware is easy and described on the OpenWrt device page (using unbricking tool nmrpflash):
If any image developer reads this thread and has some hints on how to integrate the other two versions into the OpenWrt release cycle you are highly welcome.