Take a look here @marstonstudio Strange motor reaction to using RC outputs 5-7 and other oddities
I ran into problems setting up my Feiyu 2axis gimbal also. And documented them here, It’s not very logical but i think you should be able to get it working if you read that post
You’ll have to use rc passthrough function on the RC_X channels you specify (RC_9, RC_8 etc) Pasthrough will mean RC_9 will passthrough your transmitters channel 9 signal.
Best of luck