I always use wifi for sideloading books, because my Kindle can not connect to PC with USB.
To prevent OTA updates, follow these steps: (from this post —>
https://www.mobileread.com/forums/sho...d.php?t=275877)
add the OTA blocker directory (folder) name to the top level of visible USB storage.
The blocker directory is named: update.bin.tmp.partial
Use that exact name, no additional extension, no contents.
I've never got OTA updates using this little trick though I always leave WiFi on, and it should work for you.