Reliable repositories such as FirmwareFile and HuaweiStockROM host tested flash files for the CUN-L21.
The phone will automatically detect the UPDATE.APP and start the flashing process. Use this if you can still access the dialer: Insert the SD card. Open the Phone app and dial: *#*#2846579#*#* huawei cun-l21 flash file sd card
You can find various regional builds (e.g., Poland, Android 5.1) at AZROM . Android 5.1) at AZROM .