One thing I discovered is that my USB-C OTG cable at the phone end would work when inserted one way, as OTG but if I rotated it 180 degrees it wouldn't work as OTG.
I assume the cable was not manufactured properly to the USB-C spec and I placed an index mark on the plug so I insert it the correct way every time.
Also - buy a good USB-C (male) to USB-A (female) OTG cable and verify your phone can read external USB memory sticks.
If it can, use that new OTG cable with a standard (non-OTG) USB-A to USB-C cable with your nanoVNA.
I've done that with success as well.
...Larry
