Which guide did you follow?
Did you modify ‘updateFirmwareFromFile.sh’ to downgrade firmware?
yes, look through this thread and the [GUIDE] Transmission 2.82 thread.
basically download firmware, and unzip to the public file share in nas.
ssh into nas, and after script to comment out the down-grade prevention.
run script and wait 10 mins for it to complete, and then reboot.