Hello,
Could you share a screenshot of an error, which happen during downgrade and explain the issue?
Also, could you list the problems with the latest firmware that you have?
Lastly, could you share a troubleshoot file from the device? Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.
Best regards,