Use the asus-rt-ac66u/ folder for this router (hardware revision "A1"). For download locations and other info see Where do I download firmware in the FAQ Research "New Build" and model/rev forum threads: do not blindly install a latest build without checking Read the Peacock announcement for older recommended builds and general information.
Apr 21, 2019 Asus RT-AC66U 802.11ac router review: Latest firmware Oct 30, 2012 Asus rt ac66u dd wrt firmware Full guides for Download and
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Jul 05, 2018 ASUS RT-AC66U B1 Firmware version 3.0.0.4.384.10007 Jan 15, 2018
Sep 19, 2012
ASUS RT-AC66U Firmware version 3.0.0.4.382.52272 - Fixed CVE-2018-20334 - Fixed CVE-2018-20336 - Fixed null pointer issue. RT-AC66U | Networking | ASUS USA