RT-AC68R 2.4GHz Freq. Not Connecting

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Been using only the 5GHz freq until I got a Nest Thermostat, which only uses 2.4GHz. It was unable to connect to the AC68R's 2.4GHz freq. Tried my MacBook and iPhone. None could connect to 2.4GHz. The frequency is detected and the signal strength is shown -- but will not connect. I'm using open network with MAC filtering and have quadruple checked all MAC numbers and the 2.4GHz radio is "Enabled." I spent 45 minutes on phone with ASUS Tech who was unable to solve problem. We changed control channel from 4 to 11, rebooted, installed latest firmware, rebooted some more, etc. Has anyone had a similar experience and found a solution? Ideas.
Thanks