Tech forums often host the individual prog_emmc_firehose_8953_ddr.mbn file for use with tools like QFIL, UFI Box, or UnlockTool.
Change the USB port (use USB 2.0 instead of USB 3.0), try a different USB data cable, or restart the phone back into EDL mode. Ensure you are using a verified, correctly signed Firehose file matching the CPH1803 bootloader version. Error: "Firehose Status: Fail" or "Cannot Receive Ack" download firehose oppo a3s cph1803