You lost your modem partition. Flash only the "NON-HLOS.bin" from the stock firmware via fastboot using the command: fastboot flash modem NON-HLOS.bin .
OPPO does not provide an official unlock tool like Xiaomi. You need to use an method. oppo r9s plus custom rom
In TWRP, go to Wipe > Advanced Wipe . Select Dalvik/ART Cache, System, Data, and Cache. Swipe to wipe. You lost your modem partition
: Users found ColorOS frustratingly restrictive, noting the inability to easily switch default launchers or messaging apps. oppo r9s plus custom rom
: Save all important files and secure your device’s original stock firmware as a fallback. : You will need a PC with ADB and Fastboot drivers installed. Step 1: Unlock the Bootloader