External microphone with iPad

We recently purchased a Padcaster (padcaster.com) system and an 11" iPad Pro. It comes with the USB-C to 3.5mm adapter and a splitter to go from the 3.5mm to separate headphone and microphone connections. All I can get to work using this is the headphones. Whenever I try to use an external mic it only records with the internal mic. Since the idea is for this to be like a mobile video recording studio and we want to do more green screen videos, a big benefit is for the use of external microphones. We were hoping to continue to use the Green Screen by DoInk, but could be open to another app.

Does anyone have any recommendations of apps that may allow control of microphone inputs or other tricks that may work. Thanks!