As far as i know the main problem with a downgrade from a major release to an older one is the content in the config.
This is often not compatible with the actual release so no downgrade is possible.
The firmware should be for the right model.
If it exists a V 5.2 for the 220b i see not a problem not to use it.
If you do a firmware downgrade/upgrade via TFTP you lost the old config.
You get a fresh install with factory defaults.
I must do this yesterday because my 60D was unable to boot since a sudden power failure.
In my case the 60D must be connected over WAN1 with the network.
As i see in the documents for the 220b it has only one ethernet port, so it must be connected there.
I assume a firmwareupdate over the console should be the same synonymous with the 220b.
A TFTP-Server should be running. The firmwarefile should be in the TFTP Root.
I use TFTPD32 from this site. http://tftpd32.jounin.net/
On the Console you must Type G
It then tells you on wich port you connect the Lan-Cable
Then it asks you for the TFTP Server Ip
Then it asks you for the local Ip of the Unit.
Be sure the IPs are all in the same network segment and not used.
Then it asks you for the Image filename. (it must be endet with .out)
Finally it maybe asks you how to store the new Image.
D = default B= Backup R=Run without saving
The download is then now running und if all OK (CRC) it reboots now.
The unit themselves checkes for the correct CRC
and tells this on the console if there are any issues occures.
The whole procedure was done i 5 minutes.
Be sure that you then you use the default credentials.
Also the Ip of the unit is default. Mostly 192.168.1.99
If the unit say in your case a corrupt image it could be a failure on the flashdisc.
This occasionally happens as in my case if the unit goes down suddenly due to a powerfailure.
When you download the firmware you are capable to get a Checksum for the file.
You can verify this with several tools. The Checksum must be matched wit the verified firmware.
Hope it helps you.
post edited by Chris - 2017/06/15 07:55:49