FortiAP 221C stuck in discovery AC and unable to connect to fortigate firewall

It may be a firmware incompatibility issue, you can check in the Compatibility Matrix the firmware versions of FortiAP and FortiOS.

There is also a note that applies for this AP:

*These FortiAP models and versions do not support strong ciphers. To allow connections, input the following commands in the FortiOS CLI:

For FortiOS 7.0.0:

config system global

set ssl-static-key-ciphers enable

set strong-crypto disable

end

For FortiOS 7.0.1+:

config wireless-controller global

set tunnel-mode compatible

end