I have upgraded the firmware mentioned above to a recommended v5.3.12 before applying the version 6 or newest. The ‘firmware key’ from my InControl2 account according to this (http://www.peplink.com/knowledgebase/obtaining-your-free-firmware-license-key/) may be used to unlock such firmware upgrade. I did use it but didn’t work. The error: unlock key validation failed’.
Thanks very much in advance if you’ve encountered the same before and can share how you resolved it.