Boneyard, thank you for your help and suggestions.
It is working now but I am still trying to figure out the original configuration.
Somewhat AP was originally on another subnet 192.168.20.220, and, probably, VLAN on Fortigate-60D. Cannot understand why we need a separate VLAN. I remember I've read somewhere that AP should be on VLAN, but that was, probably, about the supported devices and Fortigate-60D wireless controller configuration.
We kind of sorted out that no need to configure a wireless controller in this case.
Ok. Then I see that AP originally was configured with VLAN 6. Is it about 192.168.6.X or just coincidence? What VLAN number means, is it just a number?
When I change the original AP static 192.168.20.220 to obtain from DHCP and enabled Wi-Fi on VLAN 1 it started working but the original Wi-Fi on VLAN 6 did not work not providing IP. Do not understand it. It started working when I unchecked 801.1Q Tag and encryption from AES to Auto.
Do not understand how it worked before with these old settings.