[HELP] can't flash bootloader and modem G920FXXU2COI5 (G920F)

i want to flash bootloader and modem G920FXXU2COI5 (g920f 5.1.1) but it show error in odin..
is it posible to downgrade bootloader and modem coz my current bootloader and modem is G920FXXU3COI9 (g920f 5.1.1) ...

Help greatly appreciated - S6 G920F Bootloop after odin flash

Hello folks. Today I did a cache, system, data wipe in TWRP on my G920F. After that I tried to restore a nandroid backup, but it failed and when I tried to restart it said there was no OS installed. So the phone wouldn't boot. I figured I did something wrong so I downloaded a 5.1.1 stock rom (UK) and flashed it with odin. But after the flash the phone would just bootloop. How can I revive the phone? Tried flashing the firmware again using another version of odin and also, I did a factory reset from the stock recovery, but no dice. Any ideas? Thanks in advance