best video app for use with external mic?

I'm using a Rode VideoMic pro shotgun mic with an iphone 6 (along with a TRS to TRRS adapter) to record and post live video on Facebook. I've had some trouble with RF interference at times, and Rode suggests two things: use phone in airplane mode (which would only work if you're able to tap into wifi) and use a video app that supports an external mic input such as MoviePro or FiLMiC Pro.

I wondered if anyone has had luck with either of these apps (around $15 a piece), or has suggestions for a free, lesser expensive app that could achieve the same result.

Primary goals: eliminate RF interference and be able to adjust audio gain.

Thanks!

Lauren