With Linksys FW >= 1.1.4.173728 you must use serial connection to install DD-WRT on US EA8500 



From EA8500_1.1.4.173728_Release_Notes.txt 
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Firmware version:   1.1.4.173728
Release date:       Jul 1, 2016

- Updated based on the latest requirements of FCC 15-407

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