C5C invald image firmware

hi I have C5c its firmware is 2.3.3 when i upload new firmware like 2.8.1 a massage box show and says …invalid image firmware …please help

You will need to utilize a middle firmware 2.5.3 I think is where some of the firmware image verification stuff happened.

http://client.help.mimosa.co/client-ptmp-firmware-release-notes-c5