Smartphone support at NPS-10
The NPS-10 has a 'smartphone I / F' mode for connecting an Android phone or iPhone (iOS13.1.2 or later)
Using the NPS-10 on your smartphone is the same as using a USB memory on your smartphone.
See below for details on using USB memory with each type of phone.

For NPS-10, you need to use a USB OTG cable connected to the phone.
1. Power up NPS-10 first. If you connect the NPS-10 to a phone while the NPS-10 is powered off, the NPS-10 will attempt to charge itself using the phone's USB power.
Your phone's battery is drained and not good.
2. Use the included USB Type-C cable to connect the NPS-10 Type-C port to the Type-A socket on the USB OTG cable.
3. Then connect it to the iPhone or Android phone.
4. The connection mode selection menu appears on the NPS-10's LCD. 'Computer I/F' 'Smartphone I/F' 'Use USB Power'
5. Select 'Smartphone I/F'. In this mode, the NPS-10 operates only on its own battery and does not consume phone power for power compatibility.

The USB OTG connection of Android phone has been supported for a long time. So it is stable. Most of Android OTG cable is does its job well.
But in case of iPhone, this feature is enabled from iOS13 which is announced at Sep 2019. So it is rather early stage and some OTG cables for iPhone is not working well.
Recommend using following cable in case of you are considering using USB OTG at iPhone.