Read this if you already tested your oscilloscope / OTG cable on another phone so you are sure it is a phone related issue.

So one specific phone do not support USB OTG. Solutions:

1. Some phone like some Huawei and Realme smartphones need to turn on USB OTG in the Android settings (go to the Settings -> Additional Settings and switch on the OTG connection)
2. Need to enable OTG on the Android system. This procedure need first to root the phone, then you can edit an xml file as described here.
3. (for Huawei users) Android Settings > Developer Options > Always prompt when connecting to USB (need to disable this one)

It has been found that USB OTG can be deactivated on Android phone (until a fresh restart) when this happen:

1. USB port goes in electrical protection (too much power requested). For example for Hantek 6074 is suggested to us a special USB Type-C adapter that can power both the tablet and the oscilloscope (available models here), giving OTG functions. DO NOT use Hantek original double cable to power the oscilloscope since new USB Type-C port deactivate the OTG function when detect direct power from the USB port, putting the tablet in charging mode.
2. USB device has some wrong usb configuration. A software data error can bring USB freezing on older Android versions (found on Android 4.4 and Android 6, not visible on Android 9).