Zyxel NXC2500 experience and DHCP problem solving

One day I a friend of mine invited me to his building where Zyxcel NXC2500 wireless controller was installed.

This controller is very nice device that allow you to connect up to 8 WiFi access points which can be centrally controlled. In order to connect more access points you need to pay.

There are access point models that can be connected to this controller and in his case it was 

ZyXEL NWA5121-NI.

You can connect them directly to controller and it automatically discover all connected devices. I have never deal with such hardware and it was good experience. 
I removed 2 of NWA5121 APs from NXC2500 web interface and could not connect again. But after resetting them and after rebooting controller it automatically appeared in AP list in web interface. And default settings such as SSID and WiFi password was put on them.

But I still have a problem with connecting to WiFi with some of laptops, iphones and smartphones.

Some of the devices connected without any problem and others just after entering WiFi password can not obtain IP address. I was confused and first try to turn off password protection, thinking that it can be because of encryption type, but nothing changed. After experiments I found out that when put IP address manually was working.
FINALLY
Today I find out what was the problem.
The problem was that DHCP pool was full
So the first thing I do is to change LAN IP address to 10.20.30.1 and then, change lease time from infinite to 3 days. And my problem was solved.

And one more thing to all users of network equipments
READ LOGs in order to solve your porblems.
Everything is in LOG