Wyse AIO 5040 secondary screen not present

I have bought 6pcs of Wyse thinclients AIO 5040 , as second hand from various sources, and seems on 2 of them I can not make work the secondary screen output. In ThinOS settings, I could not find anything to disable the secondary screen, nor in the BIOS....what could be the cause of this and how to allow the secondary screen output?
It is possible it is disabled somewhere in firmwaredirectly ? (I remember to pull firmware from one thinclient and Pushed to another one, can not remember  which exact piece it was) but it could answer the question maybe. All 6pcs have the same options in ThinOS menu in "Screen" section :  Unfortunately I am unable to upload to any of these 2 Thinclients new firmware, as I get error code that wrong image is about to be uploaded ( I am really confused about all the hardware options it was produced + firmware options and its compatibility, I can not find any informations anywhere on dell website)