Why are third party apps entirely excluded from USB OTG since Android 7?
Though my phone could have Android 7, I've stuck with 6 because of compatibility with a few Xposed modules so I don't know if this helps you, but a file manager named MiXplorer has it's own OTG driver, so maybe it could r/w your OTG device. By default this is disabled, you need to turn it on in the settings.

Please let me know if it did work for you