An Ax-lite has only a 2.4 GHz radio, so 2.4 GHx AX is correct.
Then it depends on your location, you should choose a channel with the least interferences/other signals, you can use one of your Mikrotik devices:
https://help.mikrotik.com/docs/display/ROS/WiFi#WiFi-Frequencyscan
or use an Android app like wi-fi analyzer:
http://forum.mikrotik.com/t/hap-ax3-wireless-problem/172468/1
then experiment with different width, 20/40Mhz should be slightly faster but is more prone to interferences, if you need more stability go for 20 MHz.