How to disable Arduino Yun to work as Access Point and set it to work as needed?
Yun expects wifi to be working and, if it's not, it will reboot itself after 60 seconds and return in access point mode.

In order to disable this check, edit file /etc/rc.local and turn wifi-live-or-reset into #wifi-live-or-reset (with a starting #).

Once done, use LuCI (the advanced configuration panel) to configure network interfaces.