This sounds like a setup issue within your transmitter itself. I'm assuming you're using the stock Turnigy firmware on your transmitter, which I am not familiar with. You'll need to go into the transmitter menus and make sure channels 5-8 are assigned to distinct physical controls, right now it sounds like they are all mixed to a single toggle. Look at
Page 13 of this manual under the heading "Aux-CH" and see if you can modify that menu on your radio.
Also, the specs on the iA8 receiver are a bit unclear on the Hobby King website. They say it supports
four output types - regular PWM, PPM, SBus and iBus. That seems unlikely to me, not that it doesn't work, just that as usual Chinese documentation/instructions are poor. You may need to experiment with the different protocol settings in the transmitter (I don't know what they will be, I don't have this radio myself, but just try the different ones and see if that makes a difference).
I know of at least
one other user with this same radio, but he upgraded the transmitter firmware to er9x so his comments won't be of much help to you, but he indicates he did get it to work with the stock firmware also.