If you do not use Android app, the best settings is "none".
If you do use the Android app, set it manually to the interface on which you actually have internet.
In many cases (it depends on configuration) nothing changes, in some cases it creates automagically DHCP servers, routes and what not that may simply disrupt the normal working of the device.
It is Rule #5 fo a reason 
:
The twelve Rules of Mikrotik Club