@ky-fi 

Hi,

Would there be any error message on the iPhone and when we run the airplay, are you able to find the TV on the iPhone?

Please also make sure the Airplay is enabled on the TV as well;

If possible, could you please only leave the main Deco, unplug the other satellite Decos to see if the issue still existed?

Thanks a lot and wait for your reply.

@Keithmeister 

Hi, your case might be a little bit different.

I guess both your main router and Deco work as wireless routers?

If yes, you could try to change the operation mode of Deco to the access point to have a look.