Not sure if this question has been answered on another thread already but here goes.

I use the RC with OTG to my iPhone 6. Every time I connect, I have to set the keys for the Fn and C button. Is there a way for the RC to save my choices on default or do I have to set it every time i connect?

Please make sure that Spark and the RC are at the latest firmware, set the custom settings when the drone and RC are powering on.
Besides, I would suggest not using the OTG cable to connect the RC and mobile device. Please try to reset the setting with WiFi connection with the phone.
Please keep us updated if the problem persists, thank you.

I encountered the same issue too if I use the OTG cable with my iPhone 7+.