I can recommend Merlin Firmware for Asus routers. I have a Asus AC-87U with this firmware.
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Merlins Firmware
Asuswrt-merlin officially supports the following router models:
RT-N66U
RT-AC66U
RT-AC56U
RT-AC68U (including the new revision C1)
RT-AC68P (same firmware as RT-AC68U)
RT-AC87
RT-AC3200
RT-AC88U
RT-AC3100
RT-AC5300