Hi, I am new to this community and would like to share some experience and hopefully find an answer.
I have a DSL-2640B rev.E1 router which is not officially supported by neither OpenWRT or LEDE. However, I have seen some successfull attempts with previous releases of OpenWRT 15.05.01 (no wifi) and custom LEDE r4208 (working wifi with ath9k package?), both based on DSL-2740B rev.F1. (https://forum.archive.openwrt.org/viewtopic.php?id=38534)
Based on this info, I have flashed OpenWRT 15.05.01, and upgraded to OpenWRT 18.06. WiFi did not work expected, so I have compiled from latest master branch. Flashed it, too. At this point I have tried few more flashing with the same fw's and stock fw, and I was able to see the Wireless menu on LuCi. The thing is I wasn't able to use the menu because I was unaware of that wifi comes disabled if eternet exists. Anyways, I compiled and flashed LEDE r4208 as mentioned in the previous posts.
At this point, I lost wireless menu again for good. Then, I learned little bit more about this issue. I have checked dmsg output, and pci had a calibration error. After some search, it is revealed that this means ART partition is corrupted, and there isn't any way to fix it except taking from another router of same model. While I was thinking that I rushed it little bit, I deserved it probably; I have learnt that the culprit was OpenWRT 18.06...!
Okay, that's not fair. I have seen many posts about this, but still no WARNING. I hope someone just helps to find a way out of this.