Behringer U control & camera connection kit works on an ipad, no reason it cant work on an iphone.
but be careful with only using one input on one of those as you might get a left only/right only stream, so take a stereo out into the ucontrol.
Instagram Mono’s everything anyway, so maybe your stream monos everyhting, if so it doesnt matter.
Or you can use this but I’d be worried it might lost connection/crackle/distort, but I do use this to go from oscillator to tuning app on my android phone.
** edit** yeah that’ll work.