4K Video Recording direct to External Storage

Hi,

As Title suggests, I am looking for a way to record 4k video from my iPhone 12 Directly to External storage as my iPhone storage would fill up too quickly with the length of video I need to take. 

I usually use the DJI Mimo App for my Camera as I have a DJI Gimbal. I have seen a few posts about this on various forums but there is never usually a clear answer. 

I am not adverse to using other apps if needed or to get a device to do so but I need a camera with the same or more functionality that the DJI Mimo does so it can interface with my gimbal. 

If you have any recommendations on how to do this they would be greatly appreciated.