So this shouldn't be too crazy hard...
First, install the latest beta (there will be a new one in a few hours, but the one that is available right now should be good enough)
The OnePlus 6 has a USB C port. And it came with a USB A-C adapter plug.
Plug that into the phone and the dive computer cable into the other side.
Once you plug in the dive computer (and potentially switch it into PC mode or data transfer mode - I don't know if that's needed for the Leonardo) you should see a dialog on your phone, asking if you want to open this device with Subsurface-mobile.
Approve that.
Subsurface-mobile should open. I assume you have it connected to your cloud account, otherwise just use 'no cloud' for testing.
Open the download dialog (the icon in the bottom center)
Now pick the Cressi Leonardo and 'usb-serial' as connection.
Tap the Download button
Regardless of whether this works or fails, tap on the menu (lower left corner) and Help->Ask for support
This opens an email window with the log files already included. Add comments on whether this worked or not, and send the log file (because regardless if this worked or failed, I'd love to see the log)
If any of these steps make no sense, please feel free to ask. It's hard to know how much detail to include, but I can certainly explain all of this in more details and with pictures if that's useful...
THANK YOU
/D