Hm, according to https://openwrt.org/unsupported/unsupported_features the BCM4366 should be supported for both 2.4 GHz and 5 GHz bands, and AFAICT it is working for other folks: Asus RT-AC88U only 1 device per radio can be enabled - #8 by jen1 (maybe not all features 100%, but at least the basics).