It might be that you need to change some settings on the transmitter itself. I found this info on another forum:
"For Futaba 18SZ - You'll need to configure the Trainer Port function;
Linkage menu > Trainer
ACT/INH - ACT
Teacher/Student - Student
Channel mode - 8 Ch
Both stereo and mono adapter cables appear to work with the Futaba trainer port, however caution is required here. A correctly wired adapter when installed turns the Futaba 18SZ and 14SG on. "
Source and more information: https://steamcommunity.com/app/410340/discussions/1/359543951733047725/