Actually you don't need 5.1.0-11 to 9.2.0-20.
According to the latest Plailect's A9LH Guide, what you need to do is
- Boot Decrypt9WIP by 5.1.0-11's Browser
- Use CTR transfer to downgrade 5.1.0-11 to 2.1.0-4
- Install A9LH
- Restore 5.1.0-11 backup with SysNAND Restore (Keep A9LH) option