Hi
Am looking in an 'old'(2014?) dx6i manual.
Presume your tx is mode2 (throttle on left )
Flight mode switch is the smaller switch top left. Might be marked 'gear'.
I don't see any option to assign different switches to different functions , but a look at the 'monitor' page should show switch working as it were. ( hold roller down , power on tx , scroll down 3 pages to 'monitor'. )
There is a chance that the flight mode switch didn't get recognised in the gcs set up ?
Also believe you should use a acro-model-type rather than heli (?)
As hwh says , the Librepilot but is easy.
(!)
Ps if it helps , and someone can copy this to the other q mentioned above , please do