Capturing speech audio and video with a smartphone?
Were I work we used an iPhone 7 and a Samsung S8+ for an experimental web series  to capture video. The result was awesome, specially when you use stuff like the DJI Osmo Mobile 2 Smartphone Gimbal. (The series is not yet released, will updated when the editing is finished)

At first we used Bluetooth microphones but there is a lot of issues with the range and sound quality. We used the Sennheiser ew 112-p and recorded the audio on a Tascam DR-05 . This is a little expensive, an alternative will be to use another cellphone and record the audio there with a wired microphone. 

No need for a third party app, specially on the Samsung S8, where you can manually edit all the settings. You should consider the Samsung S8 has a 10 minute time limit when recording 4K video.