I pondered this question back when ASUS stopped updating the firmware for the DSL-AC88U, alas I feel the wifi chips are slightly older on the RT-AC88U (BCM4366) compared to the BCM4366e used in the DSL-AC88U and the main cpu’s are totally different, but both Broadcom so not sure in reality, but would assume it would not work.
Flash and RAM sizes are the same for both models so would be a very good exercise if you are feeling brave and would benefit many others 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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