Just check the kernel driver. In
kernel/drivers/net/wireless/rtlwifi/rtl8821ae/sw.c
it explicitly says the firmware path:
line 246:
rtlpriv->cfg->fw_name = "rtlwifi/rtl8821aefw.bin";
line 456:
MODULE_FIRMWARE("rtlwifi/rtl8821aefw.bin");
and line 106
fw_name = "rtlwifi/rtl8821aefw_wowlan.bin";
I don't see why /system/lib/firmware/rtlwifi/ failed.