Do not flash A53s firmware (CPH2135) onto A53 (CPH2121). It will hard brick your device.
Avoid sketchy forums or torrents—malicious files are common. Trusted sources include: oppo a53 firmware flash file
Do you have a different OPPO model? Check our other firmware guides for A54, A15, and Reno series. Do not flash A53s firmware (CPH2135) onto A53 (CPH2121)