The Cap AC -s whole idea is disable 4G router -s wifi and use is as a LAN gateway only, as it’s ISP owned device and I don’t want them sneaking into my local network. 
And Cap AC, at least based specs, can be router and accespoint easily.
Anyway, as I still don’t get normal speed and no fatal config issues have found, so seems the issue is Cap AC hardware related.
And this forum have a lot similar, unsolved cases, like http://forum.mikrotik.com/t/wi-fi-speed-issues-on-hap-ac-lite/102816/1