I am getting this error: "This firmware file won't work on this router. If you downloaded it from Iinksys.com it might have become corrupted. Please try downloading the file again."
I tired downloading again and the same error. I did do a checksum of the file and it was matching.
Btw I also tried to downgrade the router to stock v1.1.4.169978 and I am getting the same error. The only file that is not giving me any error is the latest stock firmware v1.1.7.182977.
Looks like its on a locked firmware from version 1.1.4.170630. Need to get a USB to TTL cable for loading other firmwares: http://mrjcd.com/EA8500_DD-WRT/
Now thinking whether to return it and get something else or buy the cable!
Try downgrading to one of the 1.1.5.x OSes. If nothing else, there are instructions on the page I linked tell you how to do the other ways if the easy way doesn't work.
I managed to implement mrjcd's instructions that OP mentioned above on one of my two EA8500 routers, but I replaced the stock firmware with DD-WRT (as did mrjcd), and implementing those superb instructions was a pain; I did this before I discovered LEDE.
I'm guessing the same procedure would work with the LEDE firmware instead of the DD-WRT firmware, but have not tried this yet.