I was able to over clock to 1200,666 using these instructions...If CFE 1.0.2.9 has been working for your overclock in an earlier version of Merlin firmware, then it's weird that by upgrading to 378.56, it stops working...
Otherwise, I would suggest you going back to CFE 1.0.2.6 or 1.0.2.7 _unless_ your AC56U came pre-installed with 1.0.2.9.
BTW, the latest guide for overclocking in 378.56 here:
https://github.com/kvic-z/goodies-asuswrt/wiki/Overclock-Asuswrt-Merlin-on-v378.56-and-beyond
Overclocking on Asuswrt-Merlin 378.56_2 (RT-AC68U and RT-N66U)
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