FAP-221C upgrade from 5.2 build 0245 to 5.4 0354 dead

Dear all expert,

I upgrade firmware of FAP-221C from 5.2 build 0245 to 5.4 0354 via fortigate. But after upgrade, the fortiap brick.

Power led and notification led are all light green, the network led and wireless led are all light orange. The fortigate cannot recognize the fortiap. 10 second reset method or any longer reset button push are useless.

FAP-221C does not have any console interface.

How can i unbrick it.

Thanks all.