If you're unsure about any step or if the process seems too risky, consider seeking help from a professional or Huawei's official support channels.
“Flashing Huawei CUN-L21 firmware via SD card requires either the stock ‘dload’ method using a signed UPDATE.APP file, or the use of MediaTek’s SP Flash Tool with an SD card as firmware storage medium. The dload method is initiated by key combination (Vol Up+Down+Power) and is limited to signed images, whereas SP Flash Tool bypasses this restriction but needs a PC.” huawei cun-l21 flash file sd card