I'm reporting my experience with the Web app in case it might be helpful to somebody.
I opened the web page on two Android devices. They are versions 5.3 and 6.0, which I know are old, but both worked with a USB memory stick so I know that the OTG cable was good. However neither one would connect to my NanoVNA. Both said "Error: Failed to open. Error: failed to claim interface". I guess the NanoVNA firmware is too old or something. The firmware works fine for me as standalone so I don't want to upgrade it at this time.
After that, I installed nanovna-saver-v0.2.2 on Windows 10. The NanoVNA connected and transferred the scan to the graphs, so that worked.
-Pete
Pete Buxton